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/config.rs

@@ -93,6 +93,13 @@ pub struct ConfigValues {
     pub statusbar_format: Option<String>,
     pub library_tabs: Option<Vec<LibraryTab>>,
     pub hide_display_names: Option<bool>,
+    pub credentials: Option<Credentials>,
+}
+
+#[derive(Serialize, Deserialize, Debug, Default, Clone)]
+pub struct Credentials {
+    pub username_cmd: Option<String>,
+    pub password_cmd: Option<String>,
 }
 
 #[derive(Serialize, Deserialize, Debug, Default, Clone)]