Bubble unknown nested directives.
Add the ability to use @at-root to move out of directives.

Closes #774
Add a test for using @extend within @at-root.

Also fix a bug exposed by this test.
Add a Map value helper.
Use value helpers in some functions that still weren't.

Closes #952
Merge branch 'at-root-directives'

Conflicts:
	doc-src/SASS_CHANGELOG.md