Adding SelectorFormat linter

I piggybacked off of the CapitalizationInSelector and NameFormat linters to create this linter, which adds a convention for all selectors (see the linter class for the regexs used).  Similar to CapitalizationInSelector, it supports `ignored_types` and `ignored_names`.

This linter partially addresses https://github.com/causes/scss-lint/issues/227.