Support project specific download commands

Projects can now define additional download commands in their
project.config file:

[plugin "project-download-commands"]
    Build = git fetch ${url} ${ref} && git checkout FETCH_HEAD && buck build ${project}
    Update = git fetch ${url} ${ref} && git checkout FETCH_HEAD && git submodule update

Change-Id: I613ff88c948e7c9a2b1616b3b846b041c0fa488c
Signed-off-by: Edwin Kempin <edwin.kempin@sap.com>