Add support for *clip.exe* for Bash on Ubuntu on Windows

[Jason A. Donenfeld]

Hi Kenny,

Thanks for your response.

> uname -r: 4.4.0-43-Microsoft
> uname -v: #1-Microsoft Wed Dec 31 14:42:53 PST 2014

Microsoft has evidently built a time machine and made a 4.4.0 before
4.4.0 existed! Surely if they can travel back in time, they can travel
into the future too. In that case, I will stop working on this, and
instead simply wait for them to bring pass compatibility back from a
future timeline in which I actually do do the work. Wait, paradox.

> uname -r: 4.4.0-43-Microsoft

So this is really unfortunate. It means the only way we have of
detecting WSL is by grepping uname -r. That seems like it won't mix
nicely with the current strategy of source "$(uname)...". I'm a bit
hesitant to bloat pass (even more) with non-standard Microsoft hacks,
especially since Windows isn't free software, but I'll see if I can
find a solution. If you have any suggestions, I'd be happy to hear
them.

> There's a (mildly disgusting) way to shove everything into the platform
> file. PowerShell is installed by default on all versions of Windows since
> Windows 7 and Windows Server 2008 R2 (both released at the end of 2009).
> Given that WSL is new for Windows 10, it sure seems like supporting WSL
> should imply we can safely expect PowerShell to be installed and available.

Great, sounds like a plan then,

Regards,
Jason