Allow ListenSlack to receive App Mention events and include user details

When using the ListenSlack processor, I often want only the events that mention my bot by name. However, as it is, the processor requires that I receive all events in the channels that have my bot, and then filter them out. We should instead allow users to receive only App Mention events.

Additionally, it would be beneficial to retrieve user details such as username instead of just the User ID.