Fixing option parsing, and removing deprecated get_opts function

Everything is now handled by argparse. This breaks the original CLI syntax but is, I think, an improvement.

The CLI now looks like this:

    retriever install sqlite mammallh [flags]

To get help on engine-specific flags, you can run e.g. `retriever install sqlite --help`. This way the help messages are specific to the selected engine, and irrelevant flags aren't shown.

Thoughts?