Add commandline parameter -pluginpath to add plugin search path

One or more commandline parameters -pluginpath allows to add custom search paths
for plugins to PluginManager::pluginPaths

Change-Id: I6d2702f1713b99000679ae577ce0978b3de51354
Reviewed-by: Orgad Shaneh <orgads@gmail.com>
Reviewed-by: Eike Ziller <eike.ziller@digia.com>