mini-ndn

Currently, Mini-NDN will not download or install copies of dependencies onto a system which has already had them installed. While this is helpful for users who have already configured their dependencies manually, it's unintuitive for those with existing installs looking to upgrade NDN dependencies or systems which are not fresh VMs which have existing NDN code on it. This should likely be either confirmed with a Y/N prompt (though the question is what the default behavior should. be) or require an explicit flag to use it.