Support Haxe ES6 output

[elsassph]

Implement #92 , support of -D js-es=6, which means generating modern ES6 code with class keyword and all.

To try it:

• checkout the repo
• run npm link at the root of haxe-modular to prepare a symlink of the repo
• run npm link haxe-modular in your project repo to symlink to this repo

[kLabz]

I wanted to test this too but I still get this error from haxe compiler for fields initialization in react components :(

Must call `super()` constructor before accessing `this` in classes derived from an extern class with constructor