implement imap savedate extension, rfc 8514

it makes a new field available on stored messages. not when they they were
received (over smtp) or appended to the mailbox (over imap), but when they were
last "saved" in the mailbox. copy/move of a message (eg to the trash) resets
the "savedate" value. this helps implement "remove messages from trash after X
days".

imapclient/parse.go

@@ -5,6 +5,7 @@ import (
 	"io"
 	"strconv"
 	"strings"
+	"time"
 )
 
 func (c *Conn) recorded() string {
@@ -686,6 +687,19 @@ func (c *Conn) xmsgatt1() FetchAttr {
 		c.xspace()
 		return FetchInternalDate(c.xquoted()) // todo: parsed time
 
+	case "SAVEDATE":
+		c.xspace()
+		var t *time.Time
+		if c.peek('"') {
+			s := c.xquoted()
+			v, err := time.Parse("_2-Jan-2006 15:04:05 -0700", s)
+			c.xcheckf(err, "parsing savedate")
+			t = &v
+		} else {
+			c.xtake("nil")
+		}
+		return FetchSaveDate{t}
+
 	case "RFC822.SIZE":
 		c.xspace()
 		return FetchRFC822Size(c.xint64())