Cmdline flag to check two jsonnet files manifest equivalently

Evaluate both files and check JSON output is the same.

This is useful when refactoring to know whether or not the changes were purely internal (good) or had some external effect (bad).

E.g. if you're a vim user:

jsonnet --compare Foo.jsonnet Foo.jsonnet~

Can output a diff of the two JSON strings.

Conflicts with --multi.