Add support for attribute selector case insensitive

[MoOx]

http://www.w3.org/TR/2013/WD-selectors4-20130502/#attribute-case

From a @ben-eb idea

i had a brilliant, yet stupid idea for a postcss module - you know in CSS4 we will get [attr=test i] - which will match insensitively?
well, you can write a module that converts that to [attr=test], [attr=Test], [attr=TEst], [attr=TESt], [attr=TEST], [attr=tEst] etc
how genius is that?

[gjjones]

Am I wrong in thinking that this would expand to 2^n variations, where n=character length? It would be an opt-in with the "i" option, but this would get out of control quick.

Is this worth the potential pitfalls?

[ben-eb]

@gjjones Yep, which is why it's a stupid idea. But it could let people try out the new case-insensitive syntax in browsers that don't support it yet. 😄

[Semigradsky]

@MoOx Hey! I have created plugin: https://github.com/Semigradsky/postcss-attribute-case-insensitive 😸

[MoOx]

Feel free to make a PR for it ;)

[MoOx]

Closed by #306