resize_image passes back destination
Clarify composited spoiler check
WebM audio thumbnails

Thumbnails for uploaded WebM videos with audio are overlayed with an image
that indicate the video contains sound. Such thumbnails count as composite
spoilers.
More accurate messages on bad WebMs
Correct config example to WEBM_AUDIO_SPOILER
StillJob for producing ffmpeg video stills

Closes #67.
Merge branch 'samurai'

* samurai:
  StillJob for producing ffmpeg video stills
  Correct config example to WEBM_AUDIO_SPOILER
  More accurate messages on bad WebMs
  WebM audio thumbnails
  Clarify composited spoiler check
  resize_image passes back destination