Fix nickcolor issues (was nickcolor-plus.pl)

- Global colouring is now a real Irssi setting!
- Fix nickcolor level 3 to work at the start of lines and the end of
  lines and a few other cases in the middle of lines.
- Change the nickcolor level 3 "we already edited this line" flag to be
  '^Dg' instead of '^C0^O'. It's cleaner.
- Fix /color list to show the color number /color uses instead of the
  number irssi uses.