Move all subcommand handlers to a module

To unclutter in main.rs a bit.

tools/yabridgectl/src/config.rs

@@ -14,6 +14,8 @@
 // You should have received a copy of the GNU General Public License
 // along with this program.  If not, see <https://www.gnu.org/licenses/>.
 
+//! Utilities for managing yabrigectl's configuration.
+
 use rayon::prelude::*;
 use serde_derive::{Deserialize, Serialize};
 use std::collections::{BTreeMap, BTreeSet};