make "bestof" command more generic

The bestof-command is limited to the trigger "bestof", obviously.
This commit makes the logic behind more generic so that it can be
used in other commands as well.

While at there, the messages being sent when an entry was added
were shorten to a simple "OK".
Merge pull request #15 from ralfth/generic-bestof

refactor(bestof): make "bestof" command more generic