More coherent filter decrement behavior and better documentation

Fixes a minor bug where the colon in the "Deft:" prompt would remain
even after deleting all characters because `deft-filter-regexp' was
set to a list with an empty string, (""), rather than nil.
Add deft-filter-decrement-word (M-DEL)
Add deft-filter-yank (C-y)
Editing of filter string in minibuffer, with history (C-c C-l)