feat(service): add 'keep aspect ratio' option

[bartholomej]

Keep Aspect Ratio

Usage

When keepAspectRatio is true then width + height parameters
become maxWidth + maxHeight parameters

Just add true as 4th parameter. It is optional and default value is false => without breaking changes.

Example

let newWidth = 2048;
let newHeight = 2048;
let keepAspectRatio = true;

this.ng2PicaService.resize([someFile], newWidth, newHeight, keepAspectRatio).subscribe((result)=>{
  //all good, result is a file
  console.info(result);
}, error =>{
  //something went wrong 
  console.error(error);
});

Temporary dist example

If you wanna try it right now, you can install it like this (temporary solution):
Just add to you package.json
"ng2-pica": "git+https://github.com/bartholomej/ng2-pica.git#keep-aspect-ratio"

Related issue

#4