Add quality and resample vars to image variations

[timomeara]

pretty simple changes that allow one to specify image quality for generated images

[codingjoe]

@timomeara thanks for the contribution. I wished you would have written a ticket first. This is the 3rd time someone proposes this feature and I have been always rejecting them, because they are JPEG only.

Types like PNG do not support the quality argument, which is why I don't want to add it. You can overwrite the renderer, to make you own JPEG only field.

[timomeara]

i hear you. pil ignores quality for png file types, so it still works w/o error.
with optimize=True compress_level (the png specific param) is also ignored which is why i didn't bother.