Implement the "negate" option

[sholladay]

I want to achieve the following command line using this project:

convert input.png -negate output.png

Presumably something like:

const fs = require('fs');
const imagemagick = require('imagemagick-native');

fs.createReadStream('input.png')
    .pipe(imagemagick.streams.convert({
        negate : true
    }))
    .pipe(fs.createWriteStream('output.png'));

[elad]

Pull requests welcome. :)