Thumbor-video-engine

* Feature: support thumbor-aws result storage

* Fixed: issue with using the still filter in conjunction with the watermark
filter. Fixes 2.

* Support source videos in quicktime/mov format. Fixes 9.

* Ensure that ``Vary: Accept`` header is returned for requests to animated
gifs that can be auto-converted to other formats when that image is
returned from result storage.

* Added an ``APP_CLASS`` ``"thumbor_video_engine.app.ThumborServiceApp"``
that ensures appropriate ``Vary: Accept`` header is returned for requests
that automatically convert animated gifs based on Accept headers.
* Added settings ``FFMPEG_GIF_AUTO_H264`` and ``FFMPEG_GIF_AUTO_H265`` that
enable auto-conversion of animated gifs to H264 or H265 (respectively) when
``video/*`` is present in a request's ``Accept`` header.
* Added custom result storage classes that ensures animated gifs auto-converted
to mp4 are stored distinctly from animated gifs or auto-webp images.

* Added ``GIFSICLE_ARGS`` setting, which allows customization of arguments
passed to the gifsicle binary.