Itch-dl

- Enforce UTF-8 usage end-to-end to prevent garbled site mirrors (partially fixes 8)

- Add support for infobox "Code license" entries (closes 6)
- Open game jam JSON files in binary mode (thanks to jackwilsdon)

Forgot to git add a new source file to 0.3.0, this one's a bit more functional.

- Added [Settings](https://github.com/DragoonAethis/itch-dl/wiki/Configuration-Files) - it's now possible to permanently set an API key instead of providing it every single time on the command line. (Using the config file is not mandatory.)
- itch-dl now depends on Pydantic for config file validation.
- The User Agent is now being set to `python-requests/{version} itch-dl/{version}` by default. This is configurable.
- Misc cleanups, typing fixes and more descriptive error messages with wiki links.

- Fixed handling Categories in title infoboxes. (Fixes 1.)
- Fixed downloading files with no size specified in their upload metadata.

First release that isn't a hack! It should work pretty well for basic scenarios (browse pages, game jams) now.