make listing/not listing of forks configurable

some people have more or less taken over projects with their forks, so
it probably makes sense to still list them among the repositories

.themes/classic/source/_includes/asides/github.html

@@ -20,6 +20,7 @@
         github.showRepos({
             user: '{{site.github_user}}',
             count: {{site.github_repo_count}},
+            skip_forks: {{site.github_skip_forks}},
             target: '#gh_repos'
         });
     });

.themes/classic/source/javascripts/github.js

@@ -18,7 +18,8 @@ github = (function(){
                 var repos = [];
                 var i;
                 for (i = 0; i < data.repositories.length; i++){
-                  if (!data.repositories[i].fork)
+                  if (options.skip_forks && !data.repositories[i].fork)
+                    continue;
                   repos.push(data.repositories[i]);
                 }
                 repos.sort(function(a, b){