add an option to choose the path suffix for 2x images

Added an option to choose the path suffix for 2x images.

I'm using _2x path suffix for my retina images, just as www.apple.com does. But this script only works if your retina image suffix is "@2x". So I added an option.

Commit e7930be
Added another option to select just some of the img tags to make their image retina, not all the images on a webpage.
I can see "travis build failed", because of querySelectorAll. Maybe travis is using an old JS engine.
https://developer.mozilla.org/en-US/docs/DOM/Document.querySelectorAll