Make Selector instance of Semigroup

[vyorkin]

Allows to write this:

(p <> a) ? do
  display block

Rendering produces:

p, a { display: block }

Resolves #80

[JordanMartinez]

Without knowing too much about this library, I'm in favor of closing this PR. I think the combinator idea itself is worth adding, but not as this implementation. Perhaps it would be its own Combinators module. AFAICT, using , to combine two Selectors together is just one possible Semigroup instance. I imagine that the separator could also be >, +, or combinators like that.

[JordanMartinez]

Closing this PR due to the reasons stated above.

[vyorkin]

using , to combine two Selectors together is just one possible Semigroup instance. I imagine that the separator could also be >, +, or combinators like that

Good point, totally agree