Support client certificate authentication

[danopia]

mTLS support for fetch() just merged into deno's development branch from denoland/deno#11721

Once that merge commit denoland/deno@dccf4cb is in a tagged release we can introduce client certificate support as seen in #5.

Unfortunately, there's still not a way of passing these configurations to a WebSocket client, which continues to keep exec/attach/portforward just a tad out of reach.

[justinmchase]

Hopefully they will decide to add the options parameter to the websocket like nodejs did, regardless of what the standards group says about it.

[danopia]

I just released v0.3.0 including this. Right now a recent Deno canary build is required. I believe Deno v1.14.0 will be the first proper release with it. In any case --unstable is required.

I'm tracking Deno support for TLS auth in WebSocketStream here: denoland/deno#11846

This library still doesn't have merged WebSocket code so no harm yet.