Rendering NGLView trajectories as images or movies

[cxhernandez]

https://github.com/arose/nglview/blob/a0814a824b13065b130044ca271ee19faad2d2aa/nglview/tests/notebooks/export_image.ipynb

This is pretty sweet. And would be useful for interactive movie-making.

@arose, @hainm: This might be a dumb question, but would it at all possible to render images independent of the notebook? Would something like a headless browser work?