Fix attribute selector parsing in almost_any_value.

Closes #1385
Don't bubble rulesets into @keyframes.

Closes #1383
Don't emit busted property-less @keyframes rules.

Closes #1391
Fix --watch.

Closes #1382
Handle --sourcemap=none when updating individual files.

Closes #1387
Avoid generating :not selectors that fail on older browsers.

Closes #1380
Fix variable assignment semantics in top-level control structures.

Closes #1386