Merge pull request #1 from x512/dev

- enhancement: ``drop_rf()`` calls ``get_factors()`` if called beforehand. 
- added: ``.drop_mkt()``, ``--nomkt``
- fix: hml_devil using a persistent cache.
- enhancement: cleanup README.md
- chore: isort imports, some error messages.
- todo: ``hml_devil_factors`` returns HML Devil. Rename HML_Devil...