add a text input in tag select dropdown for new tag creation

logic has not been implemented yet, but now there is a text input
field in the tag dropdown.
add ability to create new tags when editing a story

users can now create new tags for stories when composing. the
multiselect directive now takes an attribute, show-add, that accepts
a boolean. by default, show-add is false. when we set
show-add="true" in our markup, the directive template will contain a
text input field and a button that allows the user to add a new tag
to that story. when you add a new tag, it is automatically checked.
in order to allow the enter key to add the tag, a new directive,
"on-enter", was created that takes a function and excecutes it when
the enter key is pressed while the element containing the directive
is focused.
prevent addition of duplicate tags

previously, when you tried to add the same tag multiple times, it
wouldn't show up in the ui - but it would be stored in the DB. now,
when you add a tag that already exists, it simply checks off that
tag. if you add a tag that exists and is already checked off,
nothing happens.