Ligatures

I’ve sent a [pull request](https://github.com/fontello/svg2ttf/pull/32) to svg2ttf implementing ligature support. This means svgicons2svgfont should now start generating ligature glyphs in SVGs. The implementation is very hacky, I’m not offended if you want me to improve it, just tell me (I’m rather picky with pull requests on my own projects)…