Feature: extract only files matching a regexp

[gwern]

In dealing with a megawarc, any reasonably broad set of results will have many hits, possibly too many to hand-write dd calls to extract efficiently (see https://github.com/chfoo/warcat/issues/7 ).

It would be useful if you could pass warcat a regexp like .*foo\.wordpress\.com.* to extract all files in a megawarc dealing with a particular website. This can be approximated by telling warcat to extract all files and then deleting non-matches with find or other shell script approaches, but at the cost of far more disk IO, temporary storage, and having to work with find. (It might also be faster, aside from the disk IO reduction, depending on whether the format stores filenames and warcat can skip over all non-matching warcs. I don't know the details there.)