Add cli flag to disable indexing

[dyc3]

Sometimes I use miniserve to serve some files over the public internet temporarily, usually for file sharing. And sometimes I forget to turn off the server when I'm done. It would be nice if I could disable indexing so that its a little bit harder to enumerate all the files I have in the folder that is being served.

I'd be willing to implement this myself. Just let me know of any gotchas I should look out for.

[svenstaro]

I'm willing to accept a contribution for this. Just make sure to add a test that shows what the proper behavior is. :)