Support other archives with Memento and CDX search APIs

[Mr0grog]

There is a lot of hardcoded stuff in this package that is specific to the Internet Archive (i.e. archive.org), but there are other similar web archives that support the Memento API (standardized) and CDX search (not standardized, but common), such as webarchive.org.uk, or any project using pywb. It would be nice to support those here.

The most pleasant approach would be to be able to create a new WaybackClient with different URLs for the memento and search endpoints, but I think there’s a good chance there might be more complex differences, and there should maybe just be different client classes for the different well-known archives.