Feature request: Implement a -WorkingDirectory parameter, for symmetry with Start-Job

[mklement0]

Start-Job (now) has a -WorkingDirectory parameter that allows you to control what the script block executed in the background process sees as $PWD, the current location.

For symmetry and convenience, Start-ThreadJob should support -WorkingDirectory too.

(As an aside: For Start-Job, -WorkingDirectory $PWD can be used to make the job use the same location as the caller, for instance; that said, use of the caller's location should have been the default all along, and in the case of Start-ThreadJob it's not too late to fix that - see #46)