Migrate some settings to use Irssi settings API

[miikkas]

Some of the variables should be configurable with the Irssi settings API (such as the location of the colour file). Modify the code to allow that.

[miikkas]

Also find out whether there are some standard Irssi settings nickcolor should (but currently doesn't) respect.

Edit Found so far:

• Nickcolor implementation of /cnicks doesn't respect a certain Irssi setting