email: add canonical_url variable for list_projects

email: add canonical_url variable for list_projects

If supplied, this url can be used to add a [git clone -b <branch> <url>]
section to the list_projects output, which indicates to the user how to
clone the specified project. This can be useful to enable users to more
easily see how to start working on a project.

The canonical url is not used by aiaiai as an access point. It mainly is
useful if the local clones that aiaiai uses are setup as --mirror
clones.

Signed-off-by: Jacob Keller <jacob.e.keller@intel.com>
Signed-off-by: Artem Bityutskiy <artem.bityutskiy@linux.intel.com>