Add ability to add base64 URI images

From what I could tell, there didn't appear to be a direct way to add an image when using PDFKit from a browser. This adds the ability to feed a base64 encoded data URI to `doc.image`. I wasn't sure what the best way (or your preferred way) is to detect if we're dealing with a base64 data URI so I went with what's currently in there.

Example:
```javascript
// red_dot URI from https://en.wikipedia.org/wiki/Data_URI_scheme
var red_dot = 'data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAAUAAAAFCAYAAACNbyblAAAAHElEQVQI12P4//8/w38GIAXDIBKE0DHxgljNBAAO9TXL0Y4OHwAAAABJRU5ErkJggg==';
doc.image(red_dot, 50, 50, {
    width: 10
});
```