Use svgstore for main processing functionality

[BrianSipple]

Originally, I had written #7 and #8 with intention of these being the start for both actively maintaining broccoli-svgstore, and developing better features for it.

But then it became apparent most of this functionality was also being duplicated by other projects -- each for their respective build systems. And so the svgstore organization was born.

Behavior for #4 has already been implemented in svgstore, and I'm close on PR'ing the behavior in #7.

With all of that in mind, I think the best path for us here would be to integrate svgstore directly when those are in place, and then I'll also revisit #8 to update our dependencies and get a solid testing structure in place.