Add quote API

Sticky encryption rule, requiring that all replies to encrypted messages
are encrypted, applies only to messages with a quote now.

Co-Authored-By: B. Petersen <r10s@b44t.com>

src/param.rs

@@ -53,6 +53,9 @@ pub enum Param {
     /// For Messages
     Forwarded = b'a',
 
+    /// For Messages: quoted text.
+    Quote = b'q',
+
     /// For Messages
     Cmd = b'S',