preg: clean up and improve

[joachimmetz]

Preg is affected by winreg: clean up and improve and is in need of clean up and improvement. Use this issue to track preg clean up and improvements efforts:

• remove need for PluginsList
• Remove fake storage from preg (Also see: https://codereview.appspot.com/301940043/diff/20001/plaso/frontend/preg.py?context=10&column_width=80)