Add HTTP API for responding to messages
Clones do not have their own API server
Allow specifying sender to imitate in API
Update readme
Fix REPL and CLI
Rename repl to cli, remove single-message cli
Update readme
Allow channel join/leave, and command permissions

- Commands can be made effective only in certain circumstances.
- /join can only be issued from a channel that the bot joined at startup
- /leave can only be issued from a channel that the bot was told to join
  using /join