Flamenco: add fps, output_file_extension, and images_or_video job settings

These are all needed to use FFmpeg on the worker to render a video from
rendered image sequences.

- fps: float, the scene FPS
- images_or_video: either 'images' or 'video', depending on what's being
  output by Blender. We don't support using FFmpeg to join chunked videos
  yet.
- output_file_extension: string like '.png' or '.exr', only set when
  outputting images (since doing this for video requires a lookup table and
  isn't even being used at the moment).

CHANGELOG.md

@@ -4,6 +4,8 @@
 ## Version 1.9.5 (in development)
 
 - Fix crashing Blender when running in background mode (e.g. without GUI).
+- Flamenco: Include extra job parameters to allow for encoding a video at the end of a render
+  job that produced an image sequence.
 
 
 ## Version 1.9.4 (2018-11-01)