Expose unilateral mailbox flag changes

This is a backwards incompatible change, since it adds a variant to a
public enum.

src/parse.rs

@@ -322,8 +322,15 @@ fn handle_unilateral<'a>(
         Response::MailboxData(MailboxDatum::Recent(n)) => {
             unsolicited.send(UnsolicitedResponse::Recent(n)).unwrap();
         }
-        Response::MailboxData(MailboxDatum::Flags(_)) => {
+        Response::MailboxData(MailboxDatum::Flags(flags)) => {
-            // TODO: next breaking change:
+            unsolicited
+                .send(UnsolicitedResponse::Flags(
+                    flags
+                        .into_iter()
+                        .map(|s| Flag::from(s.to_string()))
+                        .collect(),
+                ))
+                .unwrap();
         }
         Response::MailboxData(MailboxDatum::Exists(n)) => {
             unsolicited.send(UnsolicitedResponse::Exists(n)).unwrap();

src/types/mod.rs

@@ -262,6 +262,15 @@ pub enum UnsolicitedResponse {
     /// sequence numbers 9, 8, 7, 6, and 5.
     // TODO: the spec doesn't seem to say anything about when these may be received as unsolicited?
     Expunge(Seq),
+
+    /// An unsolicited [`FLAGS` response](https://tools.ietf.org/html/rfc3501#section-7.2.6) that
+    /// identifies the flags (at a minimum, the system-defined flags) that are applicable in the
+    /// mailbox. Flags other than the system flags can also exist, depending on server
+    /// implementation.
+    ///
+    /// See [`Flag`] for details.
+    // TODO: the spec doesn't seem to say anything about when these may be received as unsolicited?
+    Flags(Vec<Flag<'static>>),
 }
 
 /// This type wraps an input stream and a type that was constructed by parsing that input stream,