Because the nit interactive interpreter and nitx might want it
![capture du 2017-11-29 14-05-07](https://user-images.githubusercontent.com/135828/
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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