Allow hot reloading the config file.

[W-Floyd]

I don't even know how this would need to be designed at the moment, but it is something we should do to allow hotplugging support.

[W-Floyd]

In order to do this, device setup should be derived directly from config, without the concept of internal/external device.

Some fields in the config should be possible to change without a full reload of the device, for example, what command is used for the data, or the frequency of polling (basically, anything that isn't a part of the discovery message). Other fields will require republishing an amended discovery message, such as name, icon, measurement type, etc (most of the config actually).

I envision we'll need some lifecycle to a device, which will need to handle spawning a few goroutines that will listen on channels for signals to act on.