Merge client support for ENABLE extension

Author: emersion

README.md

@@ -135,6 +135,7 @@ includes:
 * [CHILDREN](https://tools.ietf.org/html/rfc3348)
 * [UNSELECT](https://tools.ietf.org/html/rfc3691)
 * [APPENDLIMIT](https://tools.ietf.org/html/rfc7889)
+* [ENABLE](https://tools.ietf.org/html/rfc5161)
 
 Support for other extensions is provided via separate packages. See below.
 
@@ -147,7 +148,6 @@ wiki](https://github.com/emersion/go-imap/wiki/Using-extensions#using-client-ext
 to learn how to use them.
 
 * [COMPRESS](https://github.com/emersion/go-imap-compress)
-* [ENABLE](https://github.com/emersion/go-imap-enable)
 * [ID](https://github.com/ProtonMail/go-imap-id)
 * [IDLE](https://github.com/emersion/go-imap-idle)
 * [METADATA](https://github.com/emersion/go-imap-metadata)

client/cmd_auth.go

@@ -252,3 +252,27 @@ func (c *Client) Append(mbox string, flags []string, date time.Time, msg imap.Li
 	}
 	return status.Err()
 }
+
+// Enable requests the server to enable the named extensions. The extensions
+// which were successfully enabled are returned.
+//
+// See RFC 5161 section 3.1.
+func (c *Client) Enable(caps []string) ([]string, error) {
+	if ok, err := c.Support("ENABLE"); !ok || err != nil {
+		return nil, ErrExtensionUnsupported
+	}
+
+	// ENABLE is invalid if a mailbox has been selected.
+	if c.State() != imap.AuthenticatedState {
+		return nil, ErrNotLoggedIn
+	}
+
+	cmd := &commands.Enable{Caps: caps}
+	res := &responses.Enabled{}
+
+	if status, err := c.Execute(cmd, res); err != nil {
+		return nil, err
+	} else {
+		return res.Caps, status.Err()
+	}
+}