title: Do an export with libgit2sharp
body: <p>I'm using libgit2sharp in a .net application. I would like to fetch a directory from github. I can do this now by cloning the entire directory using Repository.Clone. But it takes a long time, and I'm fetching more files than I need. For instance the .git-repository and also a number of folders that I'm not interested in.</p>
  
  <p>Is there a way that I can export a single folder from a git repository using libgit2sharp?
  Or should I do this in another way?</p>