v0.9.8: enhance assets function

• tested and reactivated CI3 Exceptions adaption
  • added message to twiggy->set in show_exception()
• added abilty to add safe functions (auto escaped)
  • added config option register_safe_functions & register_safe_filters (default: array())
• enhance assets function
  • add abilty to create group assets
  • create assets helper to render group assets
  • added config option render_all_assets (default: false)