send-email: add sendmail email aliases format

send-email: add sendmail email aliases format

Teach send-email to read aliases in the sendmail aliases format, i.e.

	<alias>: <address|alias>[, <address|alias>...]

Examples:

	alice: Alice W Land <awol@example.com>
	bob: Robert Bobbyton <bob@example.com>
	# this is a comment
	   # this is also a comment
	chloe: chloe@example.com
	abgroup: alice, bob
	bcgrp: bob, chloe, Other <o@example.com>

 - Quoted aliases and quoted addresses are not supported.
 - Line continuations are not supported.

Warnings are printed for explicitly unsupported constructs, and any
other lines that are not matched by the parser.

Signed-off-by: Allen Hubbe <allenbh@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>