Simple-manga-downloader

Fixes
- Crash when getting some mangadex covers

New
- Mangadex covers (they also now respect the data saver option)

Fixes
- Rate limiting not actually working as it should
- Better message on rate limit error
- Mangadex results are now also sorted by volume (should help a bit with anthologies)

Code-quality
- `cover_url` renamed to `covers`

Fixes
- mangadex language selection (API changed the query parameter)
- Typo that would cause crash if downloader was ran directly

Changes
- Very slight help message changes

Internal
- Code moved to follow [black](https://black.readthedocs.io/en/stable/) formatting
- Added flake8 with plugins to catch mistakes
- Started using isort to automatically sort imports
- Added `requirements-dev.txt`

Changes
- Migrated to the new Mangadex API
- Slight output changes
- Downloader will now exit with status code 1 if config has a invalid language code

Removed
- MDHome setting

New
- Data saving setting `conf --data-saver` that will download the lower quality images (off by default)
- `--data-saver {yes(y),no(n)}` flag for `down` and `update` that overwrites the data saver setting in the config file
- `down --language` and `update --language` arguments that allow to overwrite the config language setting

Fixes
- Mangadex no longer says that there's been an error when no chapters found matching the criteria but just prints that none were found
- Crash on python 3.6 because of unsupported `required` keyword argument in `add_subparsers`

New
- MDHome setting
- basic config validation

Fixes
- mangakakalot crash
- crash when the link points to a non existing manga

Changes
- Mangadex module now also works with URLs like `/manga/ID`
- downloader now checks the chapters in ascending order (won't effect much)
- downloader now asks for a new chapter number if it's already taken (fixes 21, might cause unexpected behaviour with `-c/--check` or `-i/--ignore-input`)

Fixes
- version check now properly compares to the newest version