Add coder tunnel command

[f0ssel]

The tunnel command takes in an environment name, a remote port, and a local port and makes a TCP tunnel to connect via webrtc.

➜  coder-cli git:(cmd-tunnel) ✗ go run cmd/coder/main.go tunnel test-1 8000 7000
2021-04-08 14:24:53.660 [INFO]  <tunnel.go:108> connecting to broker    {"url": "http://demo.f0ssel.io/api/private/envagent/60663b9f-d9cf66c1a000789e2d32c431/connect?session_token=IqragYByIf-p3ovf4fX3aH4dr0i99K1Bz"}
2021-04-08 14:24:53.906 [INFO]  <tunnel.go:126> connection state changed        {"state": "connected"}
2021-04-08 14:24:53.910 [INFO]  <tunnel.go:208> your latency directly to the agent      {"ms": 0}

I've also added a --p2p flag to the coder config-ssh command, so it will edit the ssh config to use the coder tunnel instead of tradition ssh. This will allow us to dogfood this feature pretty seamlessly for devs.