Add StringQuotes linter

Add linter which enforces single or double quotes depending on the
user's configuration.

This also introduces the `source_from_range` helper, which extracts the
original source code given a `Sass::Source::Range` object.

Closes #72

Change-Id: I61a86e28c2ad8b3ee498b64e9db4447fbae911e3
Reviewed-on: https://gerrit.causes.com/32823
Tested-by: jenkins <jenkins@causes.com>
Reviewed-by: Shane da Silva <shane@causes.com>