Mode with more bad characters

[Rizhiy]

I can type some characters with greater accuracy than others. For practice, it would be nice to have a mode where difficult characters are more frequent.
I guess the easiest way to do that would be to keep track of accuracy for each character and then select words from the vocabulary which contain the most low-accuracy characters.

Ideally, you could sample words from the vocabularly where the weight for a word would be something like prod(1/char_acc).
Alternatively, if vocabulary is small can just keep track of accuracy for each word.

EDIT: I guess this is similar to #1138, but for each character.

[Miodec]

This just got picked up by someone and should be in the works: #44