Read options from config (#1668)

* Read options from config

* Fix assert in test

* Fix tests

* Fix readme to pass rst checks in Travis

* Also support .codespellrc config file

* Fix flake8 error

* CLI args override config args

* Rename tool:codespell to just codespell in config

* Fix typo in readme

* Remove unnecessary check for existance of config files (configparser already handles this case)