includepkgs directive support added (RhBug:1055910)

related to https://github.com/akozumpl/hawkey/pull/66. I am not sure whether  should I bump hawkey version. Only in case users copied config from yum that has already contained `includepkgs` directive and have earlier hawkey version will see error. The other thing is why it's not just `include`. In yum.conf it's includepkgs and include directive is not used.