A commandline option for output format

We could provide a convenience option `--wrap` and it could be used as `jsonnet --wrap std.manifestYAML file.jsonnet` and it would pass whatever `file.jsonnet` evaluates to into `std.manifestYAML` (and assume raw string output afterwards, I guess).

That would allow output in any format, including custom ones.

This way, for example `manifestINI`, which we already have, could be used in the same way
and even if someone decides they want to generate, say, nginx config, with their custom function, they could do it just as easily.

Is there a better name for this option than `--wrap`?