Add mesh.setchannel

This adds a function, `mesh.setchannel` for setting the mesh channel without having to also pass a `pan id` and `address`, as are required with `mesh.config`.

Usage: `mesh.setchannel(26)` will set the mesh channel to 26.

**Important:** You'll want all the Scouts in your Troop to change to this new channel as well, so the practical usage should be `command.all("mesh.setchannel",26)`.

Closes: #187