Renderer: add FFmpeg options

[rodlie]

This PR adds support for FFmpeg options to the renderer.

You can now add key/value for meta, codec and format.

What is supported and works depends on various variables, check the FFmpeg documentation:

• https://www.ffmpeg.org/doxygen/3.2/group__metadata__api.html
• https://trac.ffmpeg.org/wiki/Encode/H.264
• https://trac.ffmpeg.org/wiki/Encode/H.265
• https://trac.ffmpeg.org/wiki/Encode/VFX

Fixes #241