Allow executing program to retrieve credentials

* config / authentication: permit to call external program for getting credentials

You can add into your ncspot/config.toml like this:

```
creds_username = "mylogin"
creds_passeval = "pass my_pass_path_to_spot_password"
```

Or using any password manager who send your password.to stdout.
If a newline is detected it will be automatically removed

* Move credential commands into separate structure

Also add an option for a username command

* Document credential commands

---------

Co-authored-by: Henrik Friedrichsen <henrik@affekt.org>

src/main.rs

@@ -191,7 +191,17 @@ fn main() -> Result<(), String> {
                 info!("Using cached credentials");
                 c
             }
-            None => credentials_prompt(None)?,
+            None => {
+                info!("Attempting to resolve credentials via username/password commands");
+                let creds = cfg.values().credentials.clone().unwrap_or_default();
+
+                match (creds.username_cmd, creds.password_cmd) {
+                    (Some(username_cmd), Some(password_cmd)) => {
+                        authentication::credentials_eval(&username_cmd, &password_cmd)?
+                    }
+                    _ => credentials_prompt(None)?,
+                }
+            }
         }
     };