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/mod.rs

@@ -35,6 +35,10 @@ pub(crate) fn invoke(command: Command, settings: Settings) -> anyhow::Result<()>
             subcommand: Subcommand::Help { summary },
             command_name,
         } => return cmd::help::execute(&mut w, command_name, summary),
+        Command {
+            subcommand: Subcommand::Config { config_operation },
+            ..
+        } => return cmd::config::execute(&mut w, config_operation, &settings),
         Command {
             subcommand: Subcommand::Version,
             ..
@@ -90,6 +94,10 @@ pub(crate) fn invoke(command: Command, settings: Settings) -> anyhow::Result<()>
             subcommand: Subcommand::Help { .. },
             ..
         } => unreachable!(),
+        Command {
+            subcommand: Subcommand::Config { .. },
+            ..
+        } => unreachable!(),
         Command {
             subcommand: Subcommand::Version,
             ..