Add font-feature-settings CSS property

[tomconroy]

Hi, nice work on this project!

The fontkit project supports supplying a list of opentype features: https://github.com/foliojs/fontkit#fontlayoutstring-features----

This option is equivalent to the CSS property font-feature-settings so should be straightforward to translate and included in the font.layout() calls.

This would unlock many powerful features of fonts, including e.g. tabular numerals (useful for displaying numbers in a tabular/grid layout for e.g. invoices, reports, etc), as well as things like fractions, ligatures, smallcaps, stylistic sets, and many more. This would also solve #1664 via e.g. font-feature-settings: 'tnum' 1;