Add support for `proxy.tunnel`

[dbushong]

Now a setup like this will work:

driver = wd
browser = chrome
selenium.serverUrl = http://selenium.example.com::4444/wd/hub
proxy.tunnel.host = some-externally-accessible-server.example.net

• only works with driver = wd
• remote ssh server must have GatewayPorts yes in sshd_config
• if you set proxy.tunnel.host, the proxy setup will create an ssh tunnel from a random port on that host thru to proxy.port (default 4445) on localhost
• if you also set proxy.tunnel.port, it will assume you've already done the ssh and use that port in URLs
• if you set proxy.tunnel.username, that will be used for the ssh, else process.env.USER
• if you set proxy.tunnel.sshPort, that will be used instead of 22
• if you set proxy.host (just in case there's an external vs. internal hostname), that will be used in the proxy.targetUrl, else proxy.tunnel.host will be used
• there's no longer a command port on 4446 - now it just reuses the proxy app with prefix /__testium_command__

TODO: better test of ssh client - still trying to figure out how to properly write test/processes/ssh-server.js

This PR also includes some NLM updates.

[dbushong]

Supported proxy.port = 0 and have manually tested the SSH code path - will make issue to add real test for it

[jkrems]

:shipit: