REF: NDFrame describe
- [ ] closes #xxxx
- [x] tests added / passed
- [x] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry

Refactor ``NDFrame.describe`` method.

- Create module ``pandas/io/formats/describe.py``
- Delegate ``NDFrame.describe`` to function ``describe_ndframe`` in the new module
- Implement polymorphism for describing series and dataframe
- Implement strategy pattern for describing series of different kinds (categorical, numeric, timestamp)

Benefits:
- Reduce complexity in ``pandas/core/generic.py``
- Straightforward logic how to treat each datatype (reduced if/elif/else workflow)
- Enable potential for further extension