Add --env to dask-worker, support lists for --nprocs

We might consider adding an `--env` keyword to `dask-worker` that passed values down to the `env=` parameter of `Nanny`. (in the no-nanny case, this might fail).

Additionally in the case that this is used with `--nprocs`, (this is sort of hacky) it would be useful if this parameter would also support a list of inputs, so that people could pass in different environment variables for each of their processes.

This came up in https://github.com/rapidsai/dask-cuda/issues/3#issuecomment-460850875

I'm not sure how I feel about it.