Code folding headlines for erc-gitter sections.
Enable erc-gitter-compose keybindings.

- RET :: If first line starts with ` ``` `, enable `erc-gitter-compose`
  and insert closing ` ``` `.  Otherwise either send message, or insert
  newline if `erc-gitter-compose` is non-nil.
- S-RET :: Enable `erc-gitter-compose` and insert newline
- C-RET :: Always send message.  Disable `erc-gitter-compose` if
  non-nil.
- C-c c :: Insert start and end markers for code block.  Enter compose
  mode and place point between markers.
Fix hook for `erc-gitter-minor-mode` to run whenever joining a channel
on `irc.gitter.im`.