Add option to filter style properties

[ludovic]

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

[NogaMan]

looks like the first parameter node is not passed here. Even though the issue being fix does not require it, I think it's better to include just in case. WDYT?
Or maybe let's fix the README

[IDisposable]

Nice catch @NogaMan ... I think we should add the node as a parameter

[ludovic]

OK, I'll make the change

[IDisposable]

Minor issue with the test ignoring the node parameter. I will fix that when cutting the new release.