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.
PR #50 filed to fix #49.  I can add the following multiline specs to this PR if/when that has been merged: https://gist.github.com/seanwalbran/c08b7d7590d8d2c1a124