Renderer: add support for extra options

[rodlie]

Based on #240 I think adding support for "advanced" ffmpeg options would be nice.

• Add option to add meta data
• Add option to add codec key/val
• Add option to add format key/val

Keys/Values correspond with the ffmpeg console options.

I have a working core/backend, just need to make the UI code (meeeeh).

[rodlie]

The UI is done, not the best but "good enough" for now.

Will make a PR later tonight or something.