This brings together a bunch of random, partially implemented packages for support of the new(ish) Windowsconpty API - such that we can leverage theexpect style of CLI tests, but in a way that works in Linux/OSXptys and Windowsconpty.

These include:

• Vendoring thego-expect library from Netflix w/ some tweaks to work cross-platform
• Vendoring thepty cross-platform implementation fromwaypoint-plugin-sdk
• Vendoring theconpty Windows-specific implementation fromwaypoint-plugin-sdk
• Adjusting thepty interface to work withgo-expect + the cross-plat version

There were several limitations with the current packages:

• go-expect requires the sameos.File (TTY) for input / output, butconhost requires separate file handles
• conpty does not handle input, only output
• The cross-platformpty didn't expose the full set of primitives needed forconsole

Therefore, the following changes were made:

• Handling ofstdin was added to theconpty interface
• We weren't using the full extent of thego-expect interface, so some portions were removed (ie, exec'ing a process) to simplify our implementation and make it easier to extend cross-platform
• Instead ofconsole exposing just aTty, it exposes anInTty andOutTty, to help encapsulate the difference on Windows (on Linux, these point to the same pipe)

Future improvements:

• Theisatty implementation doesn't support accurate detection ofconhost pty's without an associated process. In lieu of a more robust check, I've added a--force-tty flag intended for test case use - that forces the CLI to run in tty mode.
• It seems the windows implementation doesn't support setting a deadline. This is needed for the expect.Timeout API, but isn't used by us yet.

After the all-hands + meetings this afternoon - I'll get my Win environment set up to iterate to get the tests working there.

Fixes#241