ENH: Added public accessor registrar
Adds new methods for registing custom accessors to pandas objects.

This will be helpful for implementing https://github.com/pandas-dev/pandas/issues/18767
outside of pandas.

If we accept this I'm not sure it belongs in the top-level namespace where I have it currently. What would a good home be for this be? `pd.api.extensions`?

Closes https://github.com/pandas-dev/pandas/issues/14781