send-email: address expansion for common mailers

send-email: address expansion for common mailers

mutt, gnus, pine, mailrc formats should be supported.

Testing and feedback for correctness and completeness of all formats
and support for additional formats would be good.

Nested expansions are also supported.

More than one alias file to be used.

All alias file formats must still of be the same type, though.

Two git repo-config keys are required for this
(as suggested by Ryan Anderson):

    sendemail.aliasesfile = <filename of aliases file>
    sendemail.aliasfiletype = (mutt|gnus|pine|mailrc)

Signed-off-by: Eric Wong <normalperson@yhbt.net>
Acked-by: Ryan Anderson <ryan@michonline.com>
Signed-off-by: Junio C Hamano <junkio@cox.net>