0.1.0

• add PBR shading
• add screenshot generation via parameter (-s, --screenshot <FILE>)
  • Note: currently creates a hidden window, as the "headless" mode of glutin did not work as expected
• Determine 'nice' initial camera position from bounding box of scene
  • doesn't work in all cases yet
  • glTF cameras are still ignored
• update gltf crate to 0.9.2
• remove URL parameter (HTTP handling needs to be re-implemented differently after large changes to the gltf crate)