Implement SVG merging

[axelpale]

As suggested by @b-g here, enable SVG merging in addition to the current PNG.

See svg-sprite for a possible lib to use.

[axelpale]

Notes:

• svg-sprite provides at least 5 different modes in which to generate SVG sprites. Each mode also has many configuration options.
• svg-sprite has different icon order. Incompatible with our CSS file.
• SVG can also be used inline. Convenient but non-cacheable.
• SVG Sprite VS Icon font
• Alternative to svg-sprite: svgstore

[axelpale]

Keep this issue open until the generator is able to output files for the SVG symbol method. The generator currently outputs SVG files but only for the CSS background method.