[bot] Added trigger.is_privmsg flag

This flag will make it easier to determine in a message is from a private message or otherwise.

Signed-off-by: Eldon McGuinness <eldon.mcguinness@gmail.com>
Merge pull request #391 from EldonMcGuinness/add-trigger.is_privmsg

[bot] Added trigger.is_privmsg flag