Spriteify SVG

[pnorman]

A user may have their existing symbols as SVGs, in which case build-sprite.js will ignore them.

It should be possible to generate both normal and retina sprites from a SVG marker file.

Cross-ref #822

[jfirebaugh]

I don't think we want to take on a dependency on an SVG renderer in build-sprite itself. So this will amount to either a requirement that you pre-render your SVGs to PNG (as maki does by scripting inkscape, for example), or direct SVG support in the renderer (mapbox/mapbox-gl-style-spec#222).