Add sub parameters syntaxe for filters

Some filters have sub parameters like subtitles.

subtitles=sub.srt:force_style='FontName=DejaVu Serif,PrimaryColour=&HCCFF0000'

Due to char escape policy in FilterNode, it's impossible to pass the full options like this:

filter('subtitles', force_style="FontName=DejaVu Serif,PrimaryColour=&HCCFF0000")

So I added the possibility to pass a dictionary to the kwarg argument on filter:

filter('subtitles', force_style={"FontName": "DejaVu Serif", "PrimaryColour": "&HCCFF0000"})

But I made a hack to not escape coma when a dictionary is passed to a kward argument in filter.

You can suggest me a better way to do this