Extend image spec to support replace colors

[ishubin]

This feature will be very handy for verifying the general layout of elements excluding the inner text.
e.g.:

some_object:
    image file img/some_object.png, filter replace-colors #000 #111 #222 #333 with  #fff

Or it could also support gradient replacement

some_object:
    image file img/some_object.png, filter replace-colors #000-#333 with #fff

Also it could be nice to have it replaced with dominant color by default if the with parameter is not specified

some_object:
    image file img/some_object.png, filter replace-colors #000-#333

Maybe it would be also possible to have some kind of pixel radius for color replacement. So that it not just replaces the single pixel but also the circle area around it. This way we could automatically remove small shadows of the text as well.

some_object:
    image file img/some_object.png, filter replace-colors #000-#333  with #fff radius 1