Adopt next's parameter validation

[adswa]

MetaLad commands define parameter constraints in the same way that datalad core does. The newly introduced parameter validation in datalad-next provides a more powerful re-implementation of this, though. #357 (comment) sketches what would be needed for a single command.

The hackathon and a call earlier today brought a few issues to light where parameter validation could have caught errors or problems early on, so I think metalad would benefit a lot from adopting it. However, it would bring a dependency to datalad-next, which would be new. What do you think, @christian-monch and @mih?

[christian-monch]

Good idea, let's go for it.

[adswa]

I have made a small demo in #362 for meta_extract