watchFile-webpack-plugin

* Watch file no under webpack.
* We use ajs and rt file to build react, but webpack dev server didn't watch .rt file, so finally we wirte for watching specify folder and extension.

useage
``
watchFilePlugin = require("watchfile-webpack-plugin");
...
plugins: [
new watchFilePlugin({watchFolder: "/src/components/", watchExtension: "rt"})
],
...
``

* Which watchFolder is the folder you put the special file, and watchExtension is the extension file you look.
* About ajs, please see node-async-require-loader!