Add a flag for custom certificates

I don't think a keybinding would be useful here because we can't
remove a custom certificate which is alread set anyway.
I'll add it if somebody convince me of it's usefulness.

surf.1

@@ -126,6 +126,12 @@ Prints xid to standard output. This can be used to script the browser in for
 example
 .BR xdotool(1).
 .TP
+.B -x
+Disable custom certificates.
+.TP
+.B -X
+Enable custom certificates.
+.TP
 .B \-z zoomlevel 
 Specify the
 .I zoomlevel
@@ -268,6 +274,9 @@ plugins
 .TP
 .B m M
 styles
+.TP
+.B x X
+custom certificates
 .SH INDICATORS OF WEB PAGE
 The second part of the indicators specifies modes of the web page itself.
 .SS First character: encryption