Allow passing buffer to getColor in nodejs

[m-soli]

Currently only possibility is to pass a url to getColor as far as I can tell.

When passing a buffer to getColor - getPixels throws an error of invalid file type because it expects a type param
https://github.com/scijs/get-pixels/blob/master/node-pixels.js#L130

however getColor only takes img and quality attributes - is it possible to call getColor with a third type attribute
https://github.com/lokesh/color-thief/blob/master/src/color-thief-node.js#L59

I imagine something of this sort:

function getColor(img, quality, mimeType) {
    return new Promise((resolve, reject) => {
        getPalette(img, 5, quality, mimeType) // pass this to loadImage and then to getPixels or some other solution
            .then(palette => {
                resolve(palette[0]);
            })
            .catch(err => {
                reject(err);
            })
    });

}

[m-soli]

Duplicate - PR already exists to resolve this - #174