Postdb

What's Changed
* Improved index by SylteA in https://github.com/SylteA/postDB/pull/6


**Full Changelog**: https://github.com/SylteA/postDB/compare/v0.2.3...v0.3.0

Fixed an issue in previous bug fix..

Fixed issue when creating / dropping tables.

- Fix typehint on `column_type` / `sql_type`
- typehint `Model.columns`
- optional `cascade` kwarg on `drop_table_sql()`

Added docs.

Try to fix some stuff in external project