font url() generated with absolute paths causes problems when displayed via file:// protocol

Hi,

I'm not sure whether this is a nwb or react-octicon problem.

I'm using `react-octicon` inside an nwb project, which is displayed inside a electron application.
In electron, urls are usually loaded via the `file://` protocol, which causes the fonts to not show up since the browser is trying to load them from `/build/` which is the os root :)

Generating only relative filenames `url(.)` would work for both environments.