Support the @extend directive

[kaj]

The @extend directive should be supported by rsass, and probably before release 1.0.

It seems complicated, but there's a good introduction to how it should work at https://gist.github.com/nex3/7609394 .

Some of the steps required:

• Create a dataset of rules to extend: Refactor css output. #159
• Be able to select which rules to extend: Support the sass:selector module. #130
• Implement @extend to give a "not implemented" error message: Extend is not yet supported. #173
• Implement placeholder rules (include in the css dataset but not in the output): Filter out placeholder selectors. #180
• Actually implement @extend.