Add 'git format-patch --to=' option and 'format.to' configuration variable.

Add 'git format-patch --to=' option and 'format.to' configuration variable.

Has the same functionality as the '--cc' option and 'format.cc'
configuration variable but for the "To:" email header.  Half of the code to
support this was already there.

With email the To: header usually more important than the Cc: header.

[jc: tests are by Stephen Boyd]

Signed-off-by: Steven Drake <sdrake@xnet.co.nz>
Signed-off-by: Stephen Boyd <bebarino@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>