Add Haxe syntax highlighting in docs

[zabojad]

Waiting for rouge-ruby/rouge#815 to go its way...

[zabojad]

One solution could be to switch to Gitbook and use the Gitbook Prism plugin.

I've tried Gitbook.com but it doesn't seem to work with this plugin.

There is however another possibility: host a standalone gitbook like on https://github.com/MatthijsKamstra/haxejs

Maybe use Github Wiki, like https://github.com/yahoo/react-intl/wiki?