1.0 Readiness

This issue just states my intent for what should go into a 1.0 release.

The work herein is minimal, with a light-touch extension of the built-in URL and Json plugins from migrate_plus and the core migrate modules. Its main jobs are to:

• Get your credentials to the URL handler
• Provide convenience listings of your fields for easy mapping
• Manage Airtable's 5-requests-per-second rate limits

After completing what's in the linked tickets, I will make a 1.0 release. It is unlikely that much more work will be needed.

Please let me know if you have ideas, feature requests, needs beyond this.