In order to do this we have to plumb it through the queue and the
Handler interface, but it should allow fetches of the resources
associated with a page via the IncludeRelatedScope even if it's behind
a redirect.

Introduce an interface to decouple the Enqueue functionality from the
Crawler implementation.

The output stage can now write to size-limited, rotating WARC files
using a user-specified pattern, so that output files are always
unique.

Allow users to add to the exclude regexp lists easily.

Makes it possible to retry requests for temporary HTTP errors (429,
500, etc).

Handler errors are fatal, so that an error writing the WARC output
will cause the crawl to abort.

Detect write errors (both on the database and to the WARC output) and
abort with an error message.

Also fix a bunch of harmless lint warnings.

Defaults that are more suitable to real-world site archiving.

This change allows more complex scope boundaries, including loosening
edges a bit to include related resources of HTML pages (which makes
for more complete archives if desired).

Now generates well-formed, indexable WARC files.