Support for Custom Kanji Decks

Please consider adding an option to add custom kanji decks rather than only providing the preset decks to study from. I would like to be able to export kanji learned through jpdb.io through their api and be able to study them in Ringotan. Ideally, this import would be done automatically by interfacing with a Ringotan api but any functionality for custom decks would be extremely useful. If there isn't a planned api for Ringotan, a simple interface where the user would paste all kanji to be included in the custom deck would work. Alternatively, Ringotan could read from a text file containing the kanji to be added. Another option would be to read from a sql lite file to add additional information like sections within the deck. One use case for this is that it would allow importing a deck like https://jpdb.io/anime/1579/non-non-biyori with the ability to seperate each uniquely introduced kanjis in sections for each episode similar to the pre-built decks.