Allow for custom fallback tags.

This lets you pass an optional set of fallback tags into the react jsx transpiler. This may be useful if you're using JSX for something other than ReactDOM or HTML, such as a custom XML representation.

  -- transformer --

  transform(... {
    fallbackTags: {
      MyCustomXML: {
        FooNode: true,
        BarNode: true
      }
    }
  });

  -- source --

  /** @jsx MyCustomXML */
  <FooNode><BarNode /></FooNode>

  -- compiled --

  /** @jsx MyCustomXML */
  MyCustomXML.FooNode(null, MyCustomXML.BarNode());