Add option to write time series qc movies as mp4

[Michael Waskom]

We should be able to call out to ffmpeg much like we do with convert to make gifs, but an mp4 will be smaller and easier to control to focus on specific artifacts.

[Michael Waskom]

Actually, this may be easier to do using matplotlib animation tools (which wrap ffmpeg). Look at the code I wrote for generating stimulus movies as an example.