Added hints by letters.

The new setting 'hintkeys' holds the chars to use as hint label to fire the
hints. Default value is '0123456789', but it's now possible to use letters
too.
Removed not used getHintNumber() function.
Don't clear inputbox after toggle settings.
Added note about case sensitive hint label matching (#145).
Noticed css rule for uppercase hint labels (#145).
Merge branch 'feature/letter-hints'.
Fixed duplicate hint call if filtered by label.

If hints where fired by their label, the action was done twice one tome for
the hint fired by filtering and after that because of the timeout. This issue
was introduced during the changes for the letter hints.