Pass in image to __init__ instead of filename

[davidhariri]

Sometimes you already have the image as a PIL Image object and re-reading as another Image is a wasted of memory. Currently, I'm re-implementing ColorThief like this:

# Overrides init of ColorThief to pass in the buffer from memory
class ColorThiefFromImage(ColorThief):
    def __init__(self, image):
        self.image = image

[gsornsen]

I submitted a pull request with my changes to support both. I needed it as well: #16