Using @dirname to provide load paths for SASS compilation.

This PR reorders the setting of the `@dirname` instance variable in `JAPR::Converter` so that it's available for use when the `convert` method is run. This is useful for, among other things, providing a `load_path` to SASS.

The README has also been updated with an example section on setting the `load_path` in the `convert` method, as I imagine this is a pretty common use case.