Reuse previously added images

[super-triangle]

Hello, I added in support for reusing images that have already been added into the PDF, per issue #99. I tried to follow the style conventions I noticed in the plugin already, but if I've done anything you don't like, feel free to clean up or tell me how to clean it up. :)

The new functionality can be used in the way suggested by n8v:

doc.addImage(imgData, 'JPEG', 15, 40, 180, 180, 'Octonyan');
doc.addImage('Octonyan', 'JPEG', 15, 400, 180, 180);

[chrismaximin]

+1
Thanks! Worked really well for me while adding 9 full Letter-sized pages on Google Chrome 29.0.1547.65; jsPDF was constantly crashing without that.

[MrRio]

I like this. Perhaps we need to look at the syntax for the image function though.

We could turn this into key/value pairs. For example:

doc.addImage({
  name: 'xyz', // optional
  data: 'dataurihere', // optional if name provided
  x: 0,
  y: 0,
  type: 'jpeg', // optional if detected for datauri 
  width: 100,
  height: 20
});

[super-triangle]

Hi there. I like that as a format. I won't have time in the near future to work on that; but I'll keep it in mind when I have freetime again someday. :)