API for Favorites

[resttime]

@Wessie @hirotokun
From what I learned, there doesn't exist one yet.

[ameliaikeda]

Because I'm a failure and because adding faves can only be done through Hanyuu right now. I made the faves page, but didn't make a proper API.

Working on it when I have time.

[resttime]

No need to feel pressure, the dev'ing is pretty relaxed. Issue serves more as a reminder if anything.