cargo-vendor-filterer (Vendor rust libraries based on filters)

cargo-vendor-filterer works like "cargo vendor", but supports filters
that allow excluding libraries one isn't interested in, like
libraries only needed on other platforms, etc.

NOTE:
I've included a "cargo-mkvendored.sh" script to generate filtered
vendored sources for a rust or python/rust slackbuild.  It needs to be
run from within a slackbuild directory.
