Extra filters option

[BWindey]

Implements the -e | --extra-filters proposed in #70 .

This will add the filter at the end of the program-set filters, because when testing with "negate" (#69 ), this seems to be the safer way.

Also fixed a missing comma (see fist commit)

Examples:
reStream.sh -e "avgblur=sizeX=12"

reStream.sh -e "negate"

reStream.sh -e "lutrgb=r='if(gte(val,0)*lte(val,85),255,0)':g='if(gte(val,85)*lte(val,170),255,0)':b='if(gte(val,170)*lte(val,255),255,0)'"

(I'll experiment to see if is possible to let ffmpeg convert grayscale to some simple colours on the fly)

Arguments should be comma-seperated. I don't know if it would be necessary to add a check in the shell-script to see if it is valid output, or if we would just let it fail and let the user figure out what exactly went wrong.

The reason this is a draft PR, is for the above reason, and because I altered the way brightness is controlled. Instead of setting the max colorlevels for each color to 1/8th of their max (29/255), I used the curves filter to set the threshold at 1/8 = 0.125. The result looks the same, but it reads easier. I would like feedback about that.

Closes #70

[rien]

Looking good. Thanks for the contribution!

I will wait a few days to see if the original implementer of the filter for the rm2 responds, but if not you can poke me again and I'll merge.

[BWindey]

There is now a comment explaining what curves do for ffmpeg in this scenario.

Maybe I need to specify that this closes #70 for Github to recognise it. I'll update the first comment.