Converter enhancements

[sgfn]

• Fix handling empty input files
• Fix handling spaces in filenames
• Fix incorrect pattern match on audio_contexts structure
• Add support for converting audio-only and video-only streams
• Add :crf and :resolution reencoding params for the converter
• Slightly change the structure of :thumbnails_ctx
• Update deps
• Suggest :hackney instead of :req as the HTTP client, as that's the client :ex_aws uses by default
• Reorder and label private functions in converter.ex