Add support for interpolated strings as map keys

This PR adds support for interpolated strings as map keys.

Fixes https://github.com/sass/libsass/issues/574. Specs added https://github.com/sass/sass-spec/commit/900955c09e2c4a53d56214c6cf2b40b6b5d3163f, https://github.com/sass/sass-spec/pull/111.