Add hawkey.Query(upgradable, downgradable)

In the end, I've chosen <i>upgrad<b>able</b></i> and <i>downgrad<b>able</b></i> instead of <i><b>with_</b>upgrades</i> and <i><b>with_</b>downgrades</i>. But if it's not OK with you, we can change it.