irclib: Tweak message queue priorities

- Move PRIVMSG and NOTICE from low to normal priority to eliminate
  on-connect lag
- Add CAP to high priority to match CAPAB
Misc: add ability to specify custom help URLs when 'help' is ran without arguments
Update README.md for lilybot branch
Merge remote-tracking branch 'origin/patch-3' into lilybot
Misc: handle renaming of "help" command
Merge branch 'misc/customhelp', remote-tracking branches 'upstream/testing' and 'nyuszika7h/tweak-queue-priorities' into lilybot