Fully parse selectors nested within pseudoclass selectors.
Add some failing tests for selector pseudoclass extension.
Add superselector support for selector pseudoclasses.

This also adds a new superselector test file.
Initial support for extending into selector pseudoclasses.

There are still some specificity and unification kinks to be ironed
out.
Track specificity better when extending.
Support specificity ranges for selectors like :matches().
Support simple cases of nested selector pseudoclasses.
Support placeholder selectors in selector pseudoclasses.
Add a changelog entry.
Code review changes.
Merge pull request #1271 from sass/extend-selector-pseudo

Support extending into selector pseudoclasses