Guard React mini-editors with core.useReactMiniEditors feature flag

I'm not adding a default for this one so it won't show up in the
settings view unless the user sets it in their config explicitly. I'm
hoping it won't need to be flagged for very long.

exports/atom.coffee

@@ -15,7 +15,10 @@ unless process.env.ATOM_SHELL_INTERNAL_RUN_AS_NODE
   module.exports.$ = $
   module.exports.$$ = $$
   module.exports.$$$ = $$$
-  module.exports.EditorView = require '../src/react-editor-view'
+  if atom.config.get('core.useReactMiniEditors')
+    module.exports.EditorView = require '../src/react-editor-view'
+  else
+    module.exports.EditorView = require '../src/editor-view'
   module.exports.ScrollView = require '../src/scroll-view'
   module.exports.SelectListView = require '../src/select-list-view'
   module.exports.Task = require '../src/task'