Update underscore.string to latest underscore

Recent typings for underscore use interface instead of module.
Update _.string to match.

Wrapped interface was left unimplemented due to lack of time.
Update underscore.string to latest underscore
Mix _.string into _
Re-enable tests
Allow standalone usage and fix tests.
Merge pull request #1498 from mwisnicki/patch-1

Update underscore.string to latest underscore