Add PowerShell backend support.

[timfel]

Based on the gist I mentioned in #128

Let me know if this is out of scope :)

[purcell]

Thanks @timfel! Looks fiddly! Given that just the powershell support here is more code than the rest of the package, I wouldn't really be keen to take it on, sorry. There would be a couple of options:

1. Publish exec-path-from-powershell separately, incorporating a little of the exec-path-from-shell logic so that it is self-contained.
2. Refactor exec-path-from-shell so that it is more easily extensible, and still publish exec-path-from-powershell separately to provide such an extension.

Given that both options would involve a separate package, I'd probably suggest you go with option 1 for now. Once published, I'm happy to add a note here that directs powershell users to look at your alternative.