Add CLI entrypoint to download the kernels from a metakernel

It should be possible to download all the missing kernels from a metakernel directly from the command line:

mk-download juice_crema_5_0.tm

And it should also be possible to enforce the remote and the local kernel directly:

mk-download juno.tm --remote https://naif.jpl.nasa.gov/pub/naif/JUNO/ --kernels-dir /data/JUNO/