fixed issues

1. Found and fixed a bug in circe-generate-nick-color: forgot to pass a nick as argument to itself.
2. Documented circe-color-nicks-get-nick-color-function better.
3. Replaced reduce with cl-reduce, mapcar* with cl-mapcar; checked with elisp-check.
4. Reformatted everything.
5. Made original color generation function default, to surprise users less.
6. Replaced some defuns with defsubsts to avoid unnecessary calls overhead.