enumerate() should never return quoted strings.
Update CHANGELOG.
Better support for `transparent`, including on output.
Recognize from() and to(), the ancient WebKit gradient functions.
Make linear-gradient work more better!  Now compiles Bootstrap.
Fix variable scoping to match Sass.

Turns out ChainMap wasn't quite the right thing.  Sass defines a
variable the first time it appears, and updates the same variable **in
its original scope** on subsequent assignments.
Quick test for nested scopes.