When user types /gif keyword into the chat input, a gif image will be sent instead of the command.

I know this can be achieved by using Outgoing WebHook with a custom script, but in that way, the command (like "!gif xxx") will be sent out before the gif image arrives. I prefer to send gif directly like using /gimme

I have implemented this on my own fork, and if this is accepted I'll create a pull request.