Add generic reverse template helper

Can be used to reverse the order of any slice.

Fixes #16.

main.go

@@ -4,12 +4,12 @@ import (
 	"context"
 	"flag"
 	"fmt"
-	"github.com/KyleBanks/goodreads"
 	"html/template"
 	"io/ioutil"
 	"net/http"
 	"os"
 
+	"github.com/KyleBanks/goodreads"
 	"github.com/shurcooL/githubv4"
 	"golang.org/x/oauth2"
 )
@@ -52,6 +52,7 @@ func main() {
 		"goodReadsCurrentlyReading": goodReadsCurrentlyReading,
 		/* Utils */
 		"humanize": humanized,
+		"reverse":  reverse,
 	}).Parse(string(tplIn))
 	if err != nil {
 		fmt.Println("Can't parse template:", err)

humanize.go → template.go

@@ -2,6 +2,7 @@ package main
 
 import (
 	"fmt"
+	"reflect"
 	"time"
 
 	"github.com/dustin/go-humanize"
@@ -22,3 +23,13 @@ func humanized(t interface{}) string {
 		return fmt.Sprintf("%v", t)
 	}
 }
+
+func reverse(s interface{}) interface{} {
+	n := reflect.ValueOf(s).Len()
+	swap := reflect.Swapper(s)
+	for i, j := 0, n-1; i < j; i, j = i+1, j-1 {
+		swap(i, j)
+	}
+
+	return s
+}