Get command's CWD (untested)

Command object has new method .Cwd() (string, error). The only platform
I know how to implement this for is linux (/proc/1234/cwd) and I'm
working offline right now w/o a linux install available.

Looked around but I have no idea how to implement this in Windows :/

The Hague is a nice city. And they have a shop with some great fish.
SetStartWd + unit tests
(minor) elaborate on comment in unit test
Clear status notification listeners when done

untested
Very rudimentary, fail-fast support for WD support

Mostly committing just to get /something/ out the door. It's not
complete yet, far from it.
Merge remote-tracking branch 'origin/master'