Don't delay String_Constant's that are name colours

This will allow the eval visitor to correctly coerce the string node
to a color node.
Merge pull request #695 from xzyfer/fix/map-keys-as-colors

Fix colours as map keys
keywords() should gracefully fail for a non variable arglists
Merge pull request #696 from xzyfer/fix/keywords-without-keyword-args

keywords() should gracefully fail for a non variable arglists
Implement new colors (rebeccapurple and transparent)

Fix https://github.com/sass/libsass/issues/699
Fix https://github.com/sass/libsass/issues/700