Import go-message charsets package

Enables support for non-UTF8 charsets.

Closes: https://todo.sr.ht/~sircmpwn/koushin/18

imap.go

@@ -10,6 +10,7 @@ import (
 	"github.com/emersion/go-imap"
 	imapclient "github.com/emersion/go-imap/client"
 	"github.com/emersion/go-message"
+	_ "github.com/emersion/go-message/charset"
 	"github.com/emersion/go-message/textproto"
 )