Add introducer add command

- Create a command to add an introducer (with domains).
- Add a module for escaping regular expressions used in certifications
- Add some helper functions.

src/mail.rs

@@ -20,3 +20,19 @@ pub fn to_email(rcpt: &CString) -> Option<String> {
        else { None }
     } else { None }
 }
+
+/// Checks if the parameter has a valid email format.
+pub fn is_email(email: &str) -> bool {
+    let uid = UserID::from(email);
+
+    if let Ok(Some(e)) = uid.email() {
+        e == email
+    } else {
+        false
+    }
+}
+
+/// Checks if the parameter has a valid domain format.
+pub fn is_domain(domain: &str) -> bool {
+    is_email(format!("test@{}", domain).as_str())
+}