Implement /deep/ shadow DOM selector

[mgreter]

Fixes #452 (spec-tests: sass/sass-spec#287)

[coveralls]

Coverage decreased (-0.01%) to 80.38% when pulling 5caacc3 on mgreter:bugfix/issue_452 into ff3565b on sass:master.

[xzyfer]

@mgreter I looked into this a while ago, it's much more involved than what has been done here.

For intents and purposes the /deep/ combinator acts like the direct sibling combinator except for the shadow dom. 3.2.4 Selecting Through Shadows: the /deep/ combinator.

Essentially /deep/ needs to be treated exactly the same as +.

This is a relevant specs - https://github.com/sass/sass-spec/blob/master/spec/libsass-todo-tests/scss-tests/175_test_reference_combinator_with_parent_ref/input.scss

---

Also /deep/ is a reference combinator as defined in CSS Selectors Level 4.

The reference combinator consists of two slashes with an intervening CSS qualified name, and separates two compound selectors, e.g. A /attr/ B.

We need to handle generic reference combinators, not just /deep/.

This is a relevant spec - https://github.com/sass/sass-spec/blob/master/spec/libsass-todo-tests/scss-tests/117_test_selector_interpolation_in_reference_combinator/input.scss