Support watch/subscribe functionality

[imiric]

It should be possible for Disco clients to subscribe to updates of individual keys or a group of keys.

Suggested implementation

This might need rethinking but:

• There could be a --watch or --subscribe flag on the get command.
  If the --remote flag is also specified, then the client connects to the Server-Sent Events endpoint on the server, subscribing to the requested keys. The SSE functionality should be implemented on the server first.
• The get command could also support a --tags flag, to watch a group of keys without having to specify each one. Tagging keys should be implemented first.