cli: add options --offset and --limit to notmuch show

notmuch search does not output header values. However, when browsing
through a large email corpus, it can be time saving to be able to
paginate without running notmuch show for each message/thread.

Add --offset and --limit options to notmuch show. This is inspired from
commit 796b629c3b82 ("cli: add options --offset and --limit to notmuch
search").

Update man page, shell completion and add a test case to ensure it works
as expected.

Cc: Tim Culverhouse <tim@timculverhouse.com>
Cc: Tomi Ollila <tomi.ollila@iki.fi>
Signed-off-by: Robin Jarry <robin@jarry.cc>