Haml hash attributes spacing linter

Add a linter for checking for spaces inside hash attribute braces, analogous to the Style/SpaceInsideHashLiteralBraces cop for spaces inside ruby hash literal braces.
@sds I tried this, but looks like TagNode doesn't identify multi-line hash attributes correctly (filed https://github.com/causes/haml-lint/issues/49 with details), which means this linter won't detect spacing issues inside multiline hash attributes until that's fixed.