Adds Seen command, Bit.ly,  and friendly goodbyes. Update readme.

Plugins added in the PR:

- bitly
  - Shortens urls
- seen
  - Tracks when users join/leave the channel.
  - Offers commands to list users in the channel, and find when a user was last active.
- goodbye
  - Says "goodbye" in multiple ways when someone else says goodbye to a user.