Autocomplete: add Powershell support

[dim5]

Please consider adding powershell support for autocompletion.

Based on a quick search, one could use Register-ArgumentCompleter, but I'm by no means a powershell expert.

[ajalt]

Good idea, I would welcome a PR

[dim5]

I'm no a powershell pro, so it might be a while until I have enough time to work on this. Until then it's up for grabs :)

[joffrey-bion]

As a Windows user, I would love to see that added to Clikt. I have no idea how powershell completion works, though, so I won't be able to help move this forward :/