CLN: Preliminary formatter refactor
- [x] based on discussion here: https://github.com/pandas-dev/pandas/pull/20032#pullrequestreview-101917292
- [x] tests added / passed
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry

This PR splits the various formatters into different files so that it's easier to refactor a specific formatter. 