glTF exporter: option to export only deformation bones

io_scene_gltf2/__init__.py

...nder/exp/gltf2_blender_gather_animation_channel_target.py

...f2/blender/exp/gltf2_blender_gather_animation_channels.py

io_scene_gltf2/blender/exp/gltf2_blender_gather_joints.py

io_scene_gltf2/blender/exp/gltf2_blender_gather_nodes.py

io_scene_gltf2/blender/exp/gltf2_blender_gather_skins.py