Flask-sieve

Bug fixes.
- All conditional inclusion checks were failing if the one did not include `nullable` as a rule (19)

New Features
- This allows you to set custom messages when you use manual validation.

Improvements
- Fields separated by underscores `_` now get joined with whitespace. Such as `email_address` to `email address`.

New Features
- This adds the ability to configure the default error response.

Improvements
- App registration is now done after instantiation.

Improvements
- Fix mimetype validation to check bytes by default
- Improve test coverage.

Bug fixes
- Fix image dimensions validation.

Improvements
- Improve test coverage.
- Split documentation and project dependencies.
- Create a read-the-docs configuration.

Bug fixes

Specifying a value as nullable was not being by the validations parser.