Add functionality to support channels

Added functionality to include channels in destination, using `channel_tag` argument of Pushbullet API (https://docs.pushbullet.com/v2/pushes/).

May want to change logic when both email and channel are specified.
Changed this sprintf string - should be in the same order as the PB API now. During testing prior to change, `file_name` was being passed the `tgt` argument - likely missing a %s in the previous version.

Wondered if this was related to yesterday's discussion.