Add OstreeRepo option for an out-of-band cache dir

This allows you to have a writable cache dir even for a system-owned
repository.

Closes: #250
Approved by: cgwalters

src/libostree/libostree.sym

@@ -335,4 +335,5 @@ global:
         ostree_repo_get_remote_option;
         ostree_repo_get_remote_list_option;
         ostree_repo_get_remote_boolean_option;
+        ostree_repo_set_cache_dir;
 } LIBOSTREE_2016.4;