Adding channel status command as per webpush-http2.

Adds the ability to send a GET request to a channel endpoint to check it
status. Currently the only thing returned in a 200 if the channel
exists. The spec does not say anything about the information that should
be returned. I need to ask about this.

https://tools.ietf.org/html/draft-thomson-webpush-http2-01#section-6