format-patch: --signature-file=<file>

Add an option that allows a signature file to be used with
format-patch so that signatures with newlines and other special
characters can be easily included.

  $ git format-patch -1 --signature-file=~/.signature

The config variable format.signaturefile is also provided so that it
can be added by default.

  $ git config format.signaturefile ~/.signature
  $ git format-patch -1

Signed-off-by: Jeremiah Mahler <jmmahler@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>