Enabled loading of stylesheet files from the local file system.

I needed a way to load stylesheet files when running a web app locally from the file system, which for example is relevant for natively bundled mobile web apps. In this case XMLHttpRequest returns 0 as the status code (see https://developer.mozilla.org/es/docs/XMLHttpRequest/Usar_XMLHttpRequest#Example.3A.C2.A0Non-HTTP_synchronous_request) which I explicitly allow passing in conjunction with a new configuration option I added (allowLocalFileAccess) to allow or disallow this behavior. For now I set this to be disallowed by default.