Allow required plugins

As far as I can tell, there was no way previously for an application
to register a plugin and ensure that plugin got loaded -- it would
be necessary to manually edit the config and specify the plugin be
loaded.

This is suboptimal; if third party code wishes to track third party
extensions on the blockchain, the correct way to do this is with a
plugin, and this third party build should be able to load these
required plugins regardless of whether the config lists them or not.

This commit adds a boolean parameter to application::register_plugin
which defaults to false for backwards compatibility; however, if
set to true, the plugin will automatically be enabled when the app
initializes.

libraries/app/include/graphene/app/application.hpp

@@ -49,18 +49,17 @@ namespace graphene { namespace app {
          application();
          ~application();
 
-         void set_program_options( boost::program_options::options_description& command_line_options,
-                                   boost::program_options::options_description& configuration_file_options )const;
-         void initialize(const fc::path& data_dir, const boost::program_options::variables_map&options);
-         void initialize_plugins( const boost::program_options::variables_map& options );
+         void set_program_options(boost::program_options::options_description& command_line_options,
+                                  boost::program_options::options_description& configuration_file_options)const;
+         void initialize(const fc::path& data_dir, const boost::program_options::variables_map& options);
+         void initialize_plugins(const boost::program_options::variables_map& options);
          void startup();
          void shutdown();
          void startup_plugins();
          void shutdown_plugins();
 
          template<typename PluginType>
-         std::shared_ptr<PluginType> register_plugin()
-         {
+         std::shared_ptr<PluginType> register_plugin(bool auto_load = false) {
             auto plug = std::make_shared<PluginType>();
             plug->plugin_set_app(this);
 
@@ -73,6 +72,10 @@ namespace graphene { namespace app {
                _cfg_options.add(plugin_cfg_options);
 
             add_available_plugin( plug );
+
+            if (auto_load)
+                enable_plugin(plug->plugin_name());
+
             return plug;
          }
          std::shared_ptr<abstract_plugin> get_plugin( const string& name )const;