Fix transparent svg png fallbacks
Use .svgasimg for feature detection

* It's a sync test in modernizer head
* Seems to be a better heuristic for svg support 
in background-image than svg that simply tests
for basic svg support. ( Technically there is no
way to detect for svg support in background-image
but it seems that browsers have implemented this at
the same time as svg in img)
Add an option to skip svg optimization
Update documentation