* Added support for multiple feeds
* Implemented backwards-compatibility for feed config
* Added a test for feed config backwards-compat, fixed bugs
- Fixed language config merge bug found by a test
- Adjusted two existing tests to fully check stuff related to multiple feeds
- Added a new test for backwards-compatibility of the changes
- Fixed bugs found by the newly added test
* Renamed MightBeSingle to SingleOrVec
* Made the multiple feeds config changes "loudly" backwards-incompatible
* added #[serde(deny_unknown_fields)] to front-matter, fixed problems this found in tests