Fix parsing case-sensitive attriute selectors

[Semigradsky]

Closes #378

I changed type for ignoreCase from boolean to boolean | undefined because [foo=x] and [foo=x s] selectors are not the same, because some attributes case-insensitive (https://html.spec.whatwg.org/multipage/semantics-other.html#case-sensitivity-of-selectors) by default or it depends on document mode. Also there are differences between #foo and [id=foo] selectors: in quirks mode ID selector is case-insensitive but attribute selector is case-sensitive.

[coveralls]

Coverage remained the same at 100.0% when pulling 784b1f0 on Semigradsky:377 into 2839e92 on fb55:master.

[fb55]

Thanks a lot @Semigradsky! This will have to land as a major version, which means it is probably best to bundle it with a fix for #379.