Add :mgem gem loader to load mrbgem from mgem-list ( https://github.com/mruby/mgem-list ).

tasks/mruby_build_gem.rake

@@ -52,6 +52,26 @@ module MRuby
         else
           params[:git] = "https://bitbucket.org/#{params[:bitbucket]}.git"
         end
+      elsif params[:mgem]
+        mgem_list_dir = "#{gem_clone_dir}/mgem-list"
+        mgem_list_url = 'https://github.com/mruby/mgem-list.git'
+        if File.exist? mgem_list_dir
+          git.run_pull mgem_list_dir, mgem_list_url if $pull_gems
+        else
+          FileUtils.mkdir_p mgem_list_dir
+          git.run_clone mgem_list_dir, mgem_list_url
+        end
+
+        require 'yaml'
+
+        conf_path = "#{mgem_list_dir}/#{params[:mgem]}.gem"
+        conf_path = "#{mgem_list_dir}/mruby-#{params[:mgem]}.gem" unless File.exist? conf_path
+        fail "mgem not found: #{params[:mgem]}" unless File.exist? conf_path
+        conf = YAML.load File.read conf_path
+
+        fail "unknown mgem protocol: #{conf['protocol']}" if conf['protocol'] != 'git'
+        params[:git] = conf['repository']
+        params[:branch] = conf['branch"] if conf["branch']
       end
 
       if params[:core]