Meta data for mail replies in git pw series show

[borkmann]

This sort of goes in line with issue #50, I would like a way to extract all participants the patchwork series has been sent to and dump it in git pw series show. Example for reply instructions from lore https://lore.kernel.org/bpf/20190924192934.212317-1-edumazet@google.com/#R which shows the full to/cc chain and in-reply-to ID. Ideally could be dumped in the series show. Given I can see stored 'Headers' in the patchwork UI, this info should be available, but not yet accessible via API / tooling afaik.