Support EXTRA_*_PACKAGES like dask images

[TomAugspurger]

From https://gitter.im/pangeo-data/Lobby?at=5dc6325beeb63e1a837e6f4a.

Dask does this in a https://github.com/dask/dask-docker/blob/master/base/prepare.sh.

IIUC, we may need to add the EXTRA_PIP_PACKAGES stuff to either postBuild or start https://mybinder.readthedocs.io/en/latest/config_files.html#start-run-code-before-the-user-sessions-starts. I’m not sure, but I thikn it would have to be start, since we really don’t know the value of EXTRA_PIP_PACKAGES until the container is being created.

[rabernat]

Would have been nice to get this in #90.

[cspencerjones]

I would also find this capability useful. It would be nice to be able to use/test packages without permanently adding them to the environment, going through staging, etc.