Add basic support for colours, and move commands to own module

The colour support is very fragile at the moment; background colour doesn't
work very well at all.  Commands now reside in their own file 'commands.lua'.
Added a command for reloading Lua modules on-the-fly.
Tracking of topic state, rewrite of print_to, and various minor changes

 o Adds <C-j> in chantree as shortcut for join-channel (and i_<CR> to accept it).
 o `print_to` now takes an optional `level` parameter to signify the "importance"
   of the message, and now accepts `printf`-style formatting.
 o Added /me command.
Update readme, screenshot & source code to work with Lua 5.2.