Add optional image shadow props

[seekuehe]

Description
This PR takes the shape shadow implementation and adapts it to images.
Related to Issue #986

Usage

let pptx = new PptxGenJS();
let slide = pptx.addSlide();

slide.addImage({
  x: 1,
  y: 1,
  path: "https://images.unsplash.com/photo-1506744038136-46273834b3fb?crop=entropy&cs=tinysrgb&fit=max&fm=jpg&ixid=MnwyMTIyMnwwfDF8c2VhcmNofDJ8fGxhbmRzY2FwZXxlbnwwfHx8fDE2NTg5MzE0MDM&ixlib=rb-1.2.1&q=80&w=1080",
  shadow: {
    type: "outer",
    angle: 90,
    blur: 10,
    color: "000000",
    offset: 4,
    opacity: 0.5
  }
});

pptx.writeFile({ fileName: "PptxGenJS-Sandbox.pptx" });

Sample PPT file: PptxGenJS-Sandbox.pptx

[tjinauyeung]

This view is still missing a description of the newly added slide.

[seekuehe]

PR to upstream repo: gitbrent#1147