Add a silent mode

Hi,

That allows just dumping the email addresses in a format which is
already valid in a mail header.

I've been using goobook with its gazillion Python deps + this technique to integrate it in vim:

http://atechnologyjobisnoexcuse.com/2010/04/google-contacts-in-mutt-and-vim/

This patch adds a silent mode that allows me to avoid such a shell wrapper for goocaa, please consider it for inclusion. :-)

Thanks,

Miklos