Enable WebGL

Enabling the `experimental_webgl_enabled` preference allows you to create a WebGL context from within atom-shell.

While not necessarily important for the Atom editor, for others to use atom-shell in substitution for node-webkit this is a potentially useful feature, e.g. for packaged HTML5 games.

Unless there's a reason to have this disabled, it would be nice to have it made available :)

Thanks!