Added the ability to specify a callback to invoke on each style property when copying, in order to filter the style properties to keep only the desired ones.
This can be interesting to simplify the copied element (removal of the background color for example) or not to keep the CSS variables (as Firefox or Safari does) and thus gain in performance or avoid failures on these browsers.

Sample use:

const filterStyles = (propertyName) {
    return !propertyName.startsWith('--');
}

const wrapper = document.getElementById('wrapper');
const blob =  domtoimage.toBlob(wrapper, { filterStyles: filterStyles});

Fixes #194