Alternate Communication Channels

**Is your feature request related to a problem? Please describe.**
I have an sensor far away from my house(let's say 1km distance).There is no wifi, even no AC power.
**Describe the solution you'd like**
Will esphomelib support LuRaWan , or GSM connection to server?

**Additional context**
Right now, esphomelib has organized all data to an object `mqtt_message`(include topic ,payload, qos, etc.), Then it called publish() method in `mqtt_client_` which is an AsyncMqttClient without source code, but inside depended library. Obviously the `mqtt_client` binds AsyncTcp+Wifi. I would like to name it "Channel".

GSM module connect to esp32 through UART (another channel), All communication between the esp32 and GSM module is AT-commands.

Imaging Esphomelib `mqtt component ` has a member called `channel`. Just modify the value of `channel` form `wifi` to `gsm`. The esp32 will turn to the new channel. Because I have tested the node under `channel_wifi`, so the node should be no issue on other channel. Isn't it awesome?