Allow user to specify frames_is_timestamp for .avi files

[calebweinreb]

Right now, when reading avi files, the default behavior is to assume that timestamps increment linearly throughout the whole movie. This is in contrast to .mkv files, where the timestamps are loaded explicitly and then read_frames is called with frames_is_timestamp=True.

There should also be an option to explicitly use the timestamps in avi files, similar to how it works for mkv files.