Add support for the Variant coq keyword

[ddemange]

It would be nice to have the Variant keyword recognized, and highlighted as the Inductive keyword in the output html.
Currently, this doesn't seem to be the case. The fix may be as simple as adding it to the coq_keywords set.

[xavierleroy]

Sure. Commit 466627c updates the lists of keywords and tactics, including Variant.

[ddemange]

Great, thanks!