Changes based on instructions from PR #135 (#137)

* book-store: add topics

* update "core" and "unlocked_by" variables

Changes based on instructions provided by PR #135

* modified "core" and "unlocked_by" settings

Made some adjustments after learning more about how "core" and "unlocked_by" are intended to function.  More changes may be in order once more feedback has been received.

config.json

@@ -17,8 +17,7 @@
     {
       "uuid": "0c75f779-dbf1-43f7-8f36-2e8d34fec410",
       "slug": "two-fer",
-      "core": false,
-      "unlocked_by": null,
+      "core": true,
       "difficulty": 1,
       "topics": [
         "Text formatting",
@@ -39,8 +38,7 @@
     {
       "uuid": "761e0143-163c-48ed-8bb3-5ad6a59fb69d",
       "slug": "leap",
-      "core": false,
-      "unlocked_by": null,
+      "core": true,
       "difficulty": 1,
       "topics": [
         "Integers"
@@ -61,7 +59,7 @@
       "uuid": "cf22573e-9099-4df5-86d6-02e9bf42784d",
       "slug": "raindrops",
       "core": false,
-      "unlocked_by": null,
+      "unlocked_by": "two-fer",
       "difficulty": 2,
       "topics": [
         "Text formatting",
@@ -92,8 +90,7 @@
     {
       "uuid": "a77d309f-ddd6-4700-b300-347fb7e1d916",
       "slug": "nucleotide-count",
-      "core": false,
-      "unlocked_by": null,
+      "core": true,
       "difficulty": 2,
       "topics": [
         "Dictionaries",
@@ -104,7 +101,7 @@
       "uuid": "3cc7a1b6-b613-4cf1-9318-fd2e48eac6a4",
       "slug": "etl",
       "core": false,
-      "unlocked_by": null,
+      "unlocked_by": "nucleotide-count",
       "difficulty": 2,
       "topics": [
         "Dictionaries",
@@ -115,8 +112,7 @@
     {
       "uuid": "dbf6dfed-b3c1-4bac-8d86-4584c1d9f08c",
       "slug": "grains",
-      "core": false,
-      "unlocked_by": null,
+      "core": true,
       "difficulty": 2,
       "topics": [
         "Integers",
@@ -139,7 +135,7 @@
       "uuid": "76754970-3b28-45f6-8f36-10f063b3f846",
       "slug": "beer-song",
       "core": false,
-      "unlocked_by": null,
+      "unlocked_by": "two-fer",
       "difficulty": 3,
       "topics": [
         "Text formatting",
@@ -149,8 +145,7 @@
     {
       "uuid": "4d8b5c36-3562-41a6-aaca-b5229059a009",
       "slug": "phone-number",
-      "core": false,
-      "unlocked_by": null,
+      "core": true,
       "difficulty": 3,
       "topics": [
         "Parsing",
@@ -161,7 +156,7 @@
       "uuid": "95ca4105-b3f7-42f8-a07d-8d912056ed26",
       "slug": "perfect-numbers",
       "core": false,
-      "unlocked_by": null,
+      "unlocked_by": "leap",
       "difficulty": 3,
       "topics": [
         "Integers"
@@ -171,7 +166,7 @@
       "uuid": "c89324f8-8f40-4e99-9ce2-4bf11d4d91d5",
       "slug": "binary-search",
       "core": false,
-      "unlocked_by": null,
+      "unlocked_by": "grains",
       "difficulty": 3,
       "topics": [
         "Searching",
@@ -182,8 +177,7 @@
     {
       "uuid": "6488fc37-7bd4-4854-8043-8b261554695d",
       "slug": "clock",
-      "core": false,
-      "unlocked_by": null,
+      "core": true,
       "difficulty": 3,
       "topics": [
         "Time",
@@ -193,8 +187,7 @@
     {
       "uuid": "6404ad71-4f20-4f5b-8530-8ba1578e9550",
       "slug": "triangle",
-      "core": false,
-      "unlocked_by": null,
+      "core": true,
       "difficulty": 3,
       "topics": [
         "Integers",
@@ -204,8 +197,7 @@
     {
       "uuid": "fcaf9a85-579b-4607-8d2c-b6353664502d",
       "slug": "allergies",
-      "core": false,
-      "unlocked_by": null,
+      "core": true,
       "difficulty": 4,
       "topics": [
         "Bitwise operations",
@@ -215,8 +207,7 @@
     {
       "uuid": "4fd252a7-520a-416d-b976-a8f4cf3c4516",
       "slug": "saddle-points",
-      "core": false,
-      "unlocked_by": null,
+      "core": true,
       "difficulty": 4,
       "topics": [
         "Matrices",
@@ -227,8 +218,7 @@
     {
       "uuid": "51b2fe3e-996c-41f2-86cd-c6666898c850",
       "slug": "circular-buffer",
-      "core": false,
-      "unlocked_by": null,
+      "core": true,
       "difficulty": 5,
       "topics": [
         "Queues",
@@ -239,7 +229,7 @@
       "uuid": "0648988c-0916-4651-897a-349c94c32ca8",
       "slug": "minesweeper",
       "core": false,
-      "unlocked_by": null,
+      "unlocked_by": "phone-number",
       "difficulty": 5,
       "topics": [
         "Parsing",
@@ -249,8 +239,7 @@
     {
       "uuid": "b4d9cd9c-bdc7-4485-bcce-80e4303fad51",
       "slug": "roman-numerals",
-      "core": false,
-      "unlocked_by": null,
+      "core": true,
       "difficulty": 5,
       "topics": [
         "Control-flow (loops)",
@@ -271,16 +260,20 @@
     {
       "uuid": "d83a653b-da78-4f49-9ab2-3cc947825a2b",
       "slug": "book-store",
-      "core": false,
-      "unlocked_by": null,
+      "core": true,
       "difficulty": 6,
-      "topics": null
+      "topics": [
+        "Sequences",
+        "Control-flow (loops)",
+        "Arrays",
+        "Classes"
+      ]
     },
     {
       "uuid": "f522e311-2d29-4fe2-a91c-5357ff85fde6",
       "slug": "wordy",
       "core": false,
-      "unlocked_by": null,
+      "unlocked_by": "phone-number",
       "difficulty": 7,
       "topics": [
         "Parsing",