Replace the `file_extensions` field with `file_globs`, which supports
UNIX style glob patterns powered by the `globset` crate.
This allows files with no extension (e.g. Dockerfiles) to be extracted,
by specifying a glob such as `*Dockerfile`.
One surprising aspect of this change is that the globs match against the
whole path, rather than just the file name.
This is a breaking change.
