SVG 9-slice feature request
Hi @matt-curtis - amazing work with the plugin! Unfortunately after getting very excited to find it, I believe that I had an unreasonable expectation. Everything works perfectly within Sketch, but for some reason I was really hoping that you'd be able to continue enforcing the constraints within an exported SVG. I'm trying to generate SVGs that maintain 9-slice scaling (like the same-named feature in Sketch itself, but that only works in the app as well and only applies to bitmaps).

From some digging it looks to be a wee bit tricky, but doable. [This guy](http://w3.eleqtriq.com/) seems to have figured out [the trick](http://w3.eleqtriq.com/2014/03/the-holy-grail-of-image-scaling/). Here's his [codepen example](http://codepen.io/DirkWeber/pen/mDiIs).

Is this something that the plugin could support? Thank you.
