Added BangFormat linter

Fixes https://github.com/causes/scss-lint/issues/244 by checking for space before and after the `!` ("bang"), and allows user to configure on which side(s) they want space.

It does not fully account for @ArmorDarks's examples, however: he also had issues with misspelling of the keywords and spacing between the keywords and the final semicolon. I did not check these because
- I believe the Sass parser will already throw errors with the misspellings, and
- unwanted space before the closing semicolon could be its own linter.

Let me know what you think.