Add a sorted search cheat

--sorted-search will sort the results by the number of occurrences of
the search term in the cheat (i.e., the relevance of the result). I
originally thought this would replace --search, but I chose to make it a
separate cheat because I’m not quite happy with it, for several reasons:

1. it’s a huge command, and not as easily understandable as --search –
   which matters, because the command is visible on every search
2. it prints the cheats within each “relevance group“ in reverse order
3. grep counts matching *lines*, not total matches, so that most cheats
   will only have one or two occurrences anyways