Transition from $id + $schema to just $schema, and versioning

[jaimergp]

Comes from #282 and #283.

We need to think how to transition existing schemas and make them pass validation. Maybe this is already the case, but needs to be assessed. There might not be too many published packages with menuinst v2 compat out there anyway.

This also brings the topic of versioning the schema via $schema, backwards compatibility, etc. menuninst will probably need to stay compatible with all previous schemas indefinitely.

Which versioning scheme should we use? Could be schemaver, or just good old semver.

Thoughts @conda/constructor @wolfv?

[jaimergp]

I've opened #288 and chose 1-1-0 for the next release, and 1-0-* for the previous releases in menuinst v2.0, 2.1 and 2.2.