Allow adding fonts to pebble.js projects

[unwiredben]

While the current pebble.js docs don't indicate that it has custom font support, looking at the source code indicates that if you set the text.font property to a number, it will use that as the resource ID for the font to load from the C runtime. Right now, CloudPebble doesn't support adding fonts to a pebble.js project, so this limitation should be removed and the pebble.js docs should be updated.

For code refs, see https://github.com/pebble/pebblejs/blob/master/src/simply/simply_stage.c#L73 and https://github.com/pebble/pebblejs/blob/master/src/simply/simply_stage.c#L568 for the runtime side and https://github.com/pebble/pebblejs/blob/master/src/js/ui/simply-pebble.js#L1199 for the JS encoding side.