Support &-suffix

[nex3]

@chriseppstein and I have decided that, given that @at-root #{&}-suffix doesn't work as well as we'd hoped, it's worth the downsides to support &-suffix on its own.

This should throw an error if it generates an invalid selector. This error should be clear about how that selector was constructed from the source code.

[xzyfer]

Does this mean the @at-root directive is being dropped, or simply the need to use interpolation on & ?

edit already answered https://gist.github.com/nex3/8050187

[nex3]

@at-root will not be dropped, although support for & in a script context is being delayed until the next release.

See also https://gist.github.com/nex3/8050187.