aerc: use aerc as an mbox viewer

Use Aerc as an mbox viewer. Open an mbox file from the command line in a
new tab with the mbox backend. Provide a convenient and quick way to
display emails from an mbox.

Usage: aerc mbox://<path>

where the path can either be a directory or an mbox file. If it is a
directory, every file with an .mbox suffix will be loaded as a folder.

The account config will be copied from the selected account. This allows
the answer emails in the mbox account.

Signed-off-by: Koni Marti <koni.marti@gmail.com>
Acked-by: Robin Jarry <robin@jarry.cc>

lib/socket.go

@@ -21,6 +21,7 @@ type AercServer struct {
 	logger   *log.Logger
 	listener net.Listener
 	OnMailto func(addr *url.URL) error
+	OnMbox   func(source string) error
 }
 
 func StartServer(logger *log.Logger) (*AercServer, error) {
@@ -92,6 +93,16 @@ func (as *AercServer) handleClient(conn net.Conn) {
 			} else {
 				conn.Write([]byte("result: success\n"))
 			}
+		case "mbox":
+			var err error
+			if as.OnMbox != nil {
+				err = as.OnMbox(msg)
+			}
+			if err != nil {
+				conn.Write([]byte(fmt.Sprintf("result: %v\n", err)))
+			} else {
+				conn.Write([]byte("result: success\n"))
+			}
 		}
 	}
 	as.logger.Printf("Closed Unix connection %d", clientId)