exportImage: add option to specify filename

[hainm]

currently the downloaded image's name is 'screenshot.png'. If I do the screenshot several times, Chrome adds 'screenshot(2).png' . So I need to rename it.

[arose]

I refactored the image generation (5ca95ef) and stage.makeImage now returns a Blob via a Promise. So you can trigger download with whatever name you like. You can also send the Blob-bytes to the Python kernel in nglview.

stage.makeImage( {
    factor: 1,
    antialias: true,
    trim: false,
    transparent: false
} ).then( function( blob ){
    NGL.download( blob, "screenshot.png" );
} );

[hainm]

that's perfect. Can't wait to have new NGL in NGLView.