Add option to hide generated furigana by default

[qntoine]

As discussed in #30 , it would be nice to have an option to hide generated furigana by default and display them only when e.g. hovering over (for mouse interfaces) or clicking (for touch interfaces) on a certain word.

Current behavior does not mimic an actual reading experience in Japanese (even in a learning context, e.g. a textbook or flashcards), where only readings for the most difficult or target words are shown. Displaying all furigana is distracting and can hinder one's ability to progress as the brain intuitively goes for the easiest route and hinge on furigana first.

[lukasSchulze]

Adding this (like you already mentioned in the discussion) to the CSS seems to work for me now (tested on Anki for desktop and Ankidroid)

ruby rt { visibility: hidden; }
ruby:hover rt { visibility: visible; }