[node-canvas] Provide API to list available fonts

<!--- Having trouble installing node-canvas? Please make sure you have read the installation instructions located here before asking for help: https://github.com/Automattic/node-canvas#installation Still having problems, found a bug or want a feature? Fill out the form below. --> <!--- Provide a general summary of the issue in the Title above --> ## Issue or Feature <!--- Provide info about the bug or feature. --> Is it currently possible to get a list of the available fonts? - i can't see an applicable api... It seems this should be possible in pango using pango_font_map_list_families (); I understand it's not a feature of HTML5 canvas, but we already have some extended features, including RegisterFont in node-canvas. If it's not currently possible, is it something which others are interested in? Certainly for me, as both an aid to giving a user an informed choice, and as a diagnostic aid when display is not as expected, this would be a really useful feature; and on