Add the "tls.email" directive

To receive expiration warnings from Let's Encrypt.

directives.go

@@ -103,6 +103,12 @@ func parseTLS(srv *Server, d *Directive) error {
 				return err
 			}
 			srv.ACMEManager.CA = caURL
+		case "email":
+			var email string
+			if err := child.ParseParams(&email); err != nil {
+				return err
+			}
+			srv.ACMEManager.Email = email
 		default:
 			return fmt.Errorf("unknown %q directive", child.Name)
 		}

tlstunnel.1.scd

@@ -64,6 +64,10 @@ The following directives are supported:
 	*acme_ca* <url>
 		ACME Certificate Authority endpoint.
 
+	*email* <address>
+		The email address to use when creating or selecting an existing ACME
+		server account
+
 # FILES
 
 _/etc/tlstunnel/config_