Option to download ZIP for batch downloads

[eight04]

I'm not sure how useful is this.

Good parts:

• It would benefit users who want to zip their images after downloads.
• saveAs flag is available when downloading a single zip file.

Bad parts:

• The compression can be done by advanced applications (e.g. 7-zip) which is faster and more reliable.
• Currently there are other issues (#15) making saveAs flag hard to use.

@DelKatey

[MurzNN]

Most of other image download extensions downloads multiple files in one zip archive, you can lookup the code how they do this. But downloading images as separate files like in your extension is more useful for me! Because after downloading Zip I must immediately unpack them :)