Merge: highlight with ansi color (for console)

Merge: highlight with ansi color (for console)

Because the nit interactive interpreter and nitx might want it

![capture du 2017-11-29 14-05-07](https://user-images.githubusercontent.com/135828/33395803-abb12fdc-d514-11e7-9921-75a3a1ec00bb.png)

This provides a new `AnsiHighlightVisitor` than can do stuff

Note: colors are currently hard-coded and only comments, keywords, classnames and literals are highlighted. However, since this reuse the full highlighting infrastructure, a lot of semantic things can be added.

Pull-Request: #2593