Support for SVG sprites

[SnijderC]

Hi, I implemented SVG sprites in a fork of your project, but turns out I will go another direction with it. Still I thought to drop you a few lines and share the bit of work I did in case you find it helpful. To be clear my changes aren't production ready, there even more of an experiment, so you'll find no tests. Still it works mostly.. Perhaps some changes to the generation of the SASS/CSS could would be helpful.

So anyway, here it is, as is, without warranties or guarantees: https://github.com/SnijderC/sprite-factory/commit/09c519c4e91cba96696c42cbde2a989421598389 I can make an MR is you like.