Passthrough if SNI is missing for transparent mode

When operating mitmproxy in transparent mode it would be helpful if it could be configured to automatically change all connections without SNI to passthough mode. Maybe something in the line of

--set passthrough_if_no_sni=true

The modification to the code itself is small enough. Would a pull request for that be welcome? Adding the documentation is another thing…

I think it may actually make sense to split the passthrough feature into --ignore-host and --ignore-sni, where users could match on “<none>”. Either way, a pull request that improves this functionality and also updates the docs would be more than welcome!

OK, I’ll make it something simple then. I’d rather leave the full rewrite of that part to the experts or wait until I am more familiar with the code.