Add plist to bitlbee formula

Should this run as a user other than root? (Probably.) I don't know which one to use, though. Please help me help us all! :smiley: 
It seems like most formulae only use `HOMEBREW_PREFIX` when passing a config file to the executable. (i.e. redis) In bitlbee's case, the executable is linked in the sbin folder, ending up in the user's PATH so spelling out the full path seems unnecessary. Any reason why it would be needed here and isn't needed in most others?