Add generic badges support

[asvetlov]

Like here: https://github.com/aio-libs/multidict in README.rst It should be managed by configuration like travis and codecov buttons. I would love pypi version and supported python versions badges too.

What do you think? I can provide PR if the idea makes sense.

[kennethreitz]

I don't think this is a good idea — why have a chat room for a theme?

[asvetlov]

Well, chat room is not good, I understand your position. There are too many badges in the wild, alabaster cannot provide them all.

What's about adding generic badges support? badges config parameter is a list of dictionaries, every dict has img, target and optional alt, width, height parameters -- like RST image tag. scale and align are not required IMHO. With this param user could add any badge, just like pushing a new image tag in README.

[kennethreitz]

I think that's a great idea!

[asvetlov]

Pull Request #124 is done. @kennethreitz please review.