Add CodeFile with templating support for models/$name.js

Adds the generation of the model definition script file. [See here for more info](http://apidocs.strongloop.com/loopback-boot/v/2.0.0-beta1#adding-custom-methods-to-models).

/to @bajtos or @raymondfeng 
This looks very similar to `ConfigFile.load`, it would be nice to refactor it to share this bit of code. Not necessarily in this pull request though.