Backrefs

- **NEW**: Proper support for `\N{Unicode Name}`.
- **FIX**: Incomplete escapes will not be passed through, but will instead throw an error. For instance `\p` should only be passed through if it is complete `\p{category}`. Python 3.7 will error on this if we pass it through, and Python 3.6 will generate warnings. We should just consistently fail on it for all Python versions.