Support for PropTypes

[mmircea16]

(Moved from stateless-jsx-loader repo)

Would be great if it's possible to define the PropTypes inside the html.jsx. This'll greatly improve readability of the file, as otherwise there is no way to know what kind of props can be used.

Maybe have a convention to put the PropTypes defintion on top of the file in a comment. Similar to what others are doing, e.g. in Java has comments with Annotations in them. Maybe something similar to this

(From a suggestion of @sroidl - thank you!)