Use parser.add_argument(choices=...) to simplify code.
Add option for download directory in script.

This would be helpful for, e.g. packagers, who don't want to download
things for local use, but rather get everything in place to create a
package.
Increase resolution of GSHHS for downloading.

Because the coarse level dataset is included with cartopy, using that
resolution to trigger a download does nothing. That's not really what
the download script is expected to do.
Merge pull request #459 from QuLogic/download-output-dir

Download script additions.