first draft of v2 through v4 roadmap

From #389 

One note: I would be fine with new functionality *only* being added to the `oxli` command and not to the scripts during the 2.x series.

@ctb ready for review and merge
No, I would strongly prefer avoiding this until 3.0.

Would suggest instead,

'oxli' command is now under semantic versioning, but scripts are still the advertised and preferred entry point for users; developers and workflow systems can start to trial oxli but need not switch until 3.0.

This way biologists/bionformaticians will only see that WHEN the package transitions names, you run things in a different way.