support/download: Add support to pass options directly to downloaders

support/download: Add support to pass options directly to downloaders

This adds support to pass options to the underlying command that is used
by downloader. Useful for retrieving data with server-side checking for
user login or passwords, use a proxy or use specific options for cloning
a repository via git and hg.

Signed-off-by: Romain Perier <romain.perier@free-electrons.com>
Acked-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>