Add option to configure irc message templates

This adds the option to send different irccat messages specified in the
config. Example:

    plugins:
      irccat:
        template:
          upload: "foo bar! #REDCHEF:#NORMAL %{organization}%{current_user} uploaded #GREEN%{cookbooks}#NORMAL"
Merge pull request #68 from grobie/custom-irccat-message

Add option to configure irc message templates