Add option to only spellcheck comments

[alok]

The user may not want to spellcheck strings (I don't, anyway). If they set
g:python_spellcheck_strings = 0, then they can accomplish that. This also
closes #12.

[alok]

I thought some use of exe and variables would clean up the code, but the amount of regex escaping made me decide against that.

[nfnty]

This will create an unmaintainable mess. There has to be a better way of implementing this. Is it possible to make @Spell conditional with syntax groups? Otherwise execute is the better option; regex doesn't need to be escaped when using single quotes.

[alok]

Hmm, I'd prefer to entirely remove spell checking strings since they aren't really meant to correspond strictly to actual human language (e.g keys in a dict like a, b, c).

Otherwise, I could implement something with execute. I had another branch with some work on that.