By default, don't fetch files stored in LFS

Based on git-lfs/git-lfs#2717

The purpose of this is two-fold:

(1) It keeps clones and fetches lighter-weight

(2) It keeps our LFS bandwidth down, so we're less likely to need to
upgrade our account (though that may be free for our educational
account).

.lfsconfig

@@ -0,0 +1,2 @@
+[lfs]
+        fetchexclude = *