add support for caching subsequent requests to the same resource

[sharq88]

Separated into 3 commit (white spaces makes it difficult to see what's going on)
If you have a look at this link, it makes a bit more sense what's going on...
c5faace

The problem is if you have 10 reference to the same SVG on the page, that means 10 request to the same object, which is not desired, as it causes like a waterfall effect, images showing up one by one with a little delay.

data-src was added, so it does not kick off the downloading automatically.

Feel free to merge or update. We use a different version originated from this repo, so I thought to submit a version for you, as it might affect others.

Regards
SQ

[unclechu]

@sharq88 please, keep formatting as it was, it's really hard to get what changed in the logic.