Add properties to versions

[jimporter]

This would be a dictionary of key/value pairs for each version with no built-in meaning in mike, but which can be used by themes or other tools to do fancy things.

[AdrianoKF]

After you pointed out you'd be open to contributions towards 2.0 in issue #180 (sorry for the duplicate), I'm happy to work on this issue, if you're interested.

From an implementation perspective, this seems relatively straightforward - I guess the question is about user ergonomics on the CLI. I would propose a signature like mike deploy --prop FOO=1 --prop BAR=42 [...] VERSION, where the user can pass the same option an arbitrary number of times (similar to --env in Docker). Alternatively, if more complex properties are interesting to the user, maybe allowing the props directly in JSON format (e.g., --props-json as a CLI arg, or --props-file to enable reading from a file).

[jimporter]

This is actually about 75% done on a branch already, but thanks for the offer.