Expose Formatters via API and Improve Documentation
See the discussion in https://github.com/pandas-dev/pandas/pull/22759#discussion_r218821031 - we could potentially expose the formatters via the public API and allow users to subclass them or even assign properties to them which give finer control over what gets exported.

I've had an actual use case for this for at least the `ExcelFormatter` to change the exported header formatting. Haven't had as much of a use case for other Formatters but would love input