Expose webfonts-generator ligature option
`webfonts-generator` has a [`ligature` option](https://github.com/vusion/webfonts-generator#ligature) to disable generation of ligatures from the input filenames, but it does not seem to be possible to configure this from `webfonts-loader`. This would be useful to work around a [Firefox bug](https://bugzilla.mozilla.org/show_bug.cgi?id=1634834) with long ligatures, and also to produce smaller files.