Using non data urls when running in node

[kierans]

Thanks for this handy little library.

When running in node using node-canvas I'm wanting the result of the merge to be a buffer or a stream rather than a base64 encoded string. I think it would be possible to do this with an outputType option? The rules would be

• If not provided defaults to dataURL
• If running in browser, only dataURL is a permitted output type
• Options include: dataURL, buffer, stream
• If stream then based the canvas stream method is used based on mime type
  • image/png -> createPNGStream
  • image/jpeg -> createJPEGStream
  • application/pdf -> createPDFStream
  • else error

I'm happy to help with this feature, if it will be accepted/merged. 😄