Static files `url_for` bug with Windows-like file path.

The `url_for ` for static files works based on added `static` URL rule, This rule is based on Werkzeug's `path` converter that is designed for URL path, here we need some converter for file path. In URL path, path is '/' delimited, but in file path it can be '\' delimited.

My solution is to define a custom converter class `FilePathConverter` that uses `urllib`'s `pathname2url` function in its `to_url` method.