Option to use data-src-{breakpoint-name} for sizes

This lets you use data attributes to set the breakpoint image sources.

This is useful when you can't rename or predict your file names, e.g. WordPress automatically resized images where the dimensions are added as a suffix and can vary, even with the same image size specified

e.g. 
imageA-97x220.jpg
imageB-150x220.jpg

Based on approach taken by @gabrielnau

Also added to the jQuery version and included documentation and an example