Support multiple channels in output agent-logs

[analog-cbarber]

Currently output agent-logs only allows you to turn on individual channels.
In order to turn off all channels, you have to issue a command for every channel.

It would be nicer if you could handle:

• all as a shorthand for all channels
• allow for channel ranges, e.g. 1-10
• support comma-separated lists, e.g. 1,3,5