Added a color parameter to Chilitags::draw

[qbonnard]

Chilitags::draw can now use another color than black... but it needs to be dark enough to contrast.

I needed this for a "side project", and maybe someone else needd it. I don't know if it is worth the API change...

[severin-lemaignan]

Well, this does break the ABI (but we do not care much), but not the API (since the default behaviour is the same as before, so I think we can merge.

[qbonnard]

It can also be a first step towards an overcomplete, messy API... Am I worrying too much again ?

[severin-lemaignan]

Well... if this is your (valuable) worry, then I would move the drawer API in a separate header anyway....

[qbonnard]

Well then...