Move babel settings to a .babelrc (preactjs#828)

Since the .babelrc doesn't match the package's `files` list, it won't
be included in the published package. This means that users who don't
exclude node_modules from transpilation will be able to use preact
without it requiring them to have preact's internal babel presets and
plugins installed (which isn't necessary if they're depending on the
pre-transpiled version in dist)

.babelrc

@@ -0,0 +1,23 @@
+{
+  "presets": [
+    [
+      "env",
+      {
+        "loose": true,
+        "exclude": [
+          "transform-es2015-typeof-symbol"
+        ],
+        "targets": {
+          "browsers": [
+            "last 2 versions",
+            "IE >= 9"
+          ]
+        }
+      }
+    ]
+  ],
+  "plugins": [
+    "transform-object-rest-spread",
+    "transform-react-jsx"
+  ]
+}

package.json

@@ -118,29 +118,6 @@
     "uglify-js": "^2.7.5",
     "webpack": "^2.4.1"
   },
-  "babel": {
-    "presets": [
-      [
-        "env",
-        {
-          "loose": true,
-          "exclude": [
-            "transform-es2015-typeof-symbol"
-          ],
-          "targets": {
-            "browsers": [
-              "last 2 versions",
-              "IE >= 9"
-            ]
-          }
-        }
-      ]
-    ],
-    "plugins": [
-      "transform-object-rest-spread",
-      "transform-react-jsx"
-    ]
-  },
   "greenkeeper": {
     "ignore": [
       "babel-cli",