Define new api registerRenderer

The line doc/additional-reading/code-splitting.md should be
changed to a url when/if this proposal gets accepted.

This proposal was originally desribed here:
shakacode#477

docs/api/javascript-api.md

@@ -57,4 +57,13 @@ The best source of docs is the main [ReactOnRails.js](../../node_package/src/Rea
    */
 
   authenticityHeaders(otherHeaders = {})
+
+  /**
+   * Allows registration of renderers. A renderer is a function that accept three args:
+   * props, railsContext, and domNodeId, and is responsible for rendering a component
+   * to the DOM. Not available on the server. For one possible use case see:
+   * docs/additional-reading/code-splitting.md
+   * @param renderers (key is component name, value is renderer)
+   */
+  registerRenderer(renderers)
 ```

node_package/src/ReactOnRails.js

@@ -101,6 +101,17 @@ ctx.ReactOnRails = {
     return Authenticity.authenticityHeaders(otherHeaders);
   },
 
+  /**
+   * Allows registration of renderers. A renderer is a function that accept three args:
+   * props, railsContext, and domNodeId, and is responsible for rendering a component
+   * to the DOM. Not available on the server. For one possible use case see:
+   * docs/additional-reading/code-splitting.md
+   * @param renderers (key is component name, value is renderer)
+   */
+  registerRenderer(renderers) {
+    ComponentRegistry.registerRenderer(renderers);
+  },
+
   // /////////////////////////////////////////////////////////////////////////////
   // INTERNALLY USED APIs
   // /////////////////////////////////////////////////////////////////////////////