Comicbox

- issue_count type change from decimal to int

- Fix synthesizing CBI & CIX credits data.
- Extract decimals from strings for more liberal reading.

- CBI changed `critical_rating` to decimal type from string (in CBI it's an
int).
- CIX gained the `community_rating` attribute from CIX 2.0 spec.
- `age_rating` replaces `maturity_rating` to standardize more on CIX
- `CoverArtist` replaces `Cover` to standardize more on CIX
- Better CIX `Manga` parsing
- Better CIX `BlackAndWhite` parsing
- Added CIX `AgeRating` valid schema values, unused.
- LGPL 3.0

- Update pycountry to be compatible with codex

- Comicbox now raises a specific UnsupportedArchiveTypeError for bad archives

- Option for getting the cover image at metadata load time as well.