add deflicker option

[Gsonovb]

HI ,

I see doc deflicker has options , but can't set it .
so "Do not actually modify frame"

[11.59 deflicker](http://www.ffmpeg.org/ffmpeg-filters.html#toc-deflicker)
Remove temporal frame luminance variations.

It accepts the following options:

size, s
Set moving-average filter size in frames. Default is 5. Allowed range is 2 - 129.

mode, m
Set averaging mode to smooth temporal luminance variations.

Available values are:

‘’am
Arithmetic mean

‘’gm
Geometric mean

‘’hm
Harmonic mean

‘’qm
Quadratic mean

‘’cm
Cubic mean

‘’pm
Power mean

‘’median
Median

bypass
Do not actually modify frame. Useful when one only wants metadata.