Add locals add and remove subcommands

- `locals add` adds a local (unintroduced) certificate for encryption.
- `locals remove` removes/deacivates a local certificate.
- Overall cleanup

src/main.rs

@@ -1,6 +1,5 @@
 //! Husk milter
 
-use anyhow;
 use std::process;
 use std::sync::{Arc, Mutex};