Allow importing generic highlights, maybe even via API

[mammuth]

It might be useful to support other import formats like blank-line separated quotes in a text file.

Design hint: Might make sense to use polymorphic models?

Import sources:

• ~~Line separated text files~~ (implemented)
• Pocket (has no API for highlights 😞)
• Instapaper (#7)
• Wallabag
• RSS Feed (might work for many providers, eg. Instapaper provides one)
• Libby by Overdrive
• ... <your favorite service here!>

[mammuth]

I just implemented the possibility to import text files where highlights are separated by blank lines (I used such a format in my pre-kindle times).