PluginManager: Implement -load %plugin% option

This loads a plugin that is disabled by default.

Change-Id: Ibbc9849c417519904fe1e69a46f93a7cc1c7edc8
Reviewed-by: Christian Kandeler <christian.kandeler@digia.com>

src/libs/extensionsystem/pluginspec_p.h

@@ -75,6 +75,8 @@ public:
     QList<PluginDependency> dependencies;
     bool enabledInSettings;
     bool disabledIndirectly;
+    bool forceEnabled;
+    bool forceDisabled;
 
     QString location;
     QString filePath;