Add .exe to commands on Windows

Windows cares about extensions. This branch adds .exe suffix to all commands
Merge pull request #244 from gabriel-samfira/windows-ensure-symlinks

Add .exe to commands on Windows

On windows, executable commands must have one of the extensions found in $env:PATHEXT. This branch creates a newCommand map where names contain the .exe extension.
WIP: Runhook now validates action params.  Still needs tests.
WIP: uniter_test on worker now writes actions.yaml
WIP Action validation fails correctly in tests, need more