Allow users to export/import archive configurations

[machawk1]

This is first dependent on client-side archiving being implemented. Let's also identify the existing archive specifications (e.g., MemGator's archives.json), determine if there is some de facto "standard", and utilize what's out there.

Some questions that will likely go beyond these specs:

• How do we encode precedence, i.e., order of requests?
• How do we encode short circuiting? How should the criteria be represented?

[machawk1]

See also http://labs.mementoweb.org/aggregator_config/archivelist.xml , linked from https://timetravel.mementoweb.org/guide/api/