Minor yank plugin cleanup

- No need for the ReaderPlugin import
- Command should be registered to taglist so it's killed properly
- logging left in as an example, even though it's unused
Allow story tags/state to be toggled when prefixed with %

For example, `item-state %marked .` would toggle the marked status of
the selected item. Or `tag-item %favorite .` would toggle the
user:favorite tag.
Allow conditionals in pre and post

I realized that the ternaries didn't work on sub values. So far this has
only been used for stuff like %selected_end etc. where it's probably ok
to disallow them (they're more intended for flat escapes like %B).

Of course that's really crappy to expose that to a plugin where you
might want more "advanced" features so, for now, make pre and post tags
a special case.

Tags settings are untested, but identical to the story code.
Add favorites plugin

This is more a proof on concept than a fully implemented plugin.
Implements 'favorite' and binds it to *. Also updates the story format
to add a * in front of the title of favorited items.