Add ligature support to Node engine

[pauln]

Although svgicons2svgfont added support for ligatures several versions ago, grunt-webfont still depends on v1.1.0 (see #251) - so the Node engine is unable to support ligatures. This PR updates to the current (at time of writing) version - v8.0.0 - and passes through the relevant metadata to generate ligatures (if the ligatures option is enabled).

I haven't updated the readme in this PR; if you'd like me to include such an update, please let me know what version number you'd release this change in (so that I can list ligatures as supported in the Node engine from that version onwards).

Closes #251