cli/show: produce "email" element in sigstatus

cli/show: produce "email" element in sigstatus

When the certificate that signs a message is known to be valid, GMime
is capable of reporting on the e-mail address embedded in the
certificate.

We pass this information along to the caller of "notmuch show", as
often only the e-mail address of the certificate has actually been
checked/verified.

Furthermore, signature verification should probably at some point
compare the e-mail address of the caller against the sender address of
the message itself.  Having to parse what gmime thinks is a "userid"
to extract an e-mail address seems clunky and unnecessary if gmime
already thinks it knows what the e-mail address is.

See id:878s41ax6t.fsf@fifthhorseman.net for more motivation and discussion.

Signed-off-by: Daniel Kahn Gillmor <dkg@fifthhorseman.net>