Monster command commit

This is pretty mashed up commit, but it's all on a theme of tweaking the
commands and most changes are minor even if there are too many of them
jammed in here.

- :help command (which is where this all started) that can list
  commands and provide detailed help on them (:help <command>)

- :del now takes a natural tag (instead of a # from :list), only works
  on feeds, should probably work on arbitrary tags eventually

- :categories :categorize :show-category :remove-category category
  system to group feeds into categories - this seems big, but it's
mostly blowing the dust off of old commands with new names.

- commands are now grouped for the purposes of help output

- fixed hang in between configuration removing a tag and a DELTAG event
  happening

- fixed :bind combo commands

- fixed :bind not querying properly
Proper locking on window add / del

The callbacks in screen.py need to hold sync_lock while they mess with
window lists and do refresh.

The GraphicalLog class holds it before the call, since it needs to check
if there's already an InfoBox.

pause / unpause callbacks now also hang sync_lock
Add autocmd plugin

Issue commands on startup.

This is a little rough. I'd prefer if none of the commands generated
infobox output, but there currently isn't a mechanism for that and it's
not very important.