Implement variable substitution for Windows machines

[Schottkyc137]

See #278

Now also allows variable substitution in Windows using the %VAR_NAME% syntax

[Schottkyc137]

@Xcodo do you think that this is would work for Windows users?

[Xcodo]

Looks reasonable to me, thanks for implementing that.

I had a look at the conversation in the subst package too. I don't think %VAR_NAME% is absolutely required. %{VAR_NAME} would also likely be fine for most people as long as it is documented.

[Schottkyc137]

I'll merge this for now just to be able to roll the fix for windows out. But this will likely not a very long-lived change, depending on what subst does.