Refactor for more pluggable interface

[cboettig]

• Refactor query_history() and query_sources() to be more easily extensible
• Add Zenodo into the list of default registries

This means that content identifiers can be resolved against Zenodo sources as well. Note that at this time, Zenodo only supports md5-based checksums, for which collisions are possible (though unlikely).

contentid:::sources_zenodo("hash://md5/e27c99a7f701dab97b7d09c467acf468")

                                   identifier                                                                               source       date  size status                                         md5 sha1 sha256 sha384 sha512
1 hash://md5/e27c99a7f701dab97b7d09c467acf468 https://zenodo.org/api/files/5967f986-b599-4492-9a08-94ce32323dc2/vostok.icecore.co2 2020-02-22 11036    200 hash://md5/e27c99a7f701dab97b7d09c467acf468 <NA>   <NA>   <NA>   <NA>

Consequently, functions such as resolve() or query_sources() can now successfully resolve to content found in the Zenodo archive.

Hopefully at some point Zenodo will consider sha256