emacs: make "+" and "-" tagging operations in notmuch-show more flexible

emacs: make "+" and "-" tagging operations in notmuch-show more flexible

Before the change, "+" and "-" tagging operations in notmuch-show view
accepted only a single tag.  The patch makes them use the recently
added `notmuch-read-tag-changes' function, which allows to enter
multiple tags with "+" and "-" prefixes.  So after the change, "+" and
"-" bindings in notmuch-show view allow to both add and remove
multiple tags.  The only difference between "+" and "-" is the
minibuffer initial input ("+" and "-" respectively).