Separate sass engine and cli into separate gems

[Natalie Weizenbaum]

From sass/sass#1996:

Proposal

Gems:

• sass-core will be the Sass Engine and supporting ruby code.
• sass will be cli and depend on sass-core.

Git:

• sass-core will be fork of sass git repo.
• sass will have the core code removed and remain the main git repo.

Existing Issues:

• Leave them in sass/sass

New issues:

• either repo is fine. we won't police users about it.

Versioning:

• a release to sass-core will force a release of sass and vice versa keeping version numbers in sync.

[Natalie Weizenbaum]

I'm closing this issue as "on ice" because Ruby Sass is deprecated and no additional features are planned for it.