Add a ta config set subcommand

This uses `toml_edit` to edit the config file in-place.  For the moment,
it only supports top-level arguments, but can be extended to do other
things later.

cli/src/invocation/cmd/mod.rs

@@ -1,6 +1,7 @@
 //! Responsible for executing commands as parsed by [`crate::argparse`].
 
 pub(crate) mod add;
+pub(crate) mod config;
 pub(crate) mod gc;
 pub(crate) mod help;
 pub(crate) mod info;