Also resolve path of src/href attributes.

This PR makes referencing to local files (including images) work with the `basedir` option, just like the include/extend commands do.

This is based on two assumptions:

1. Escaped attributes, in this case "src" and "href", always start and end with double quotes  
    (I couldn't find a clean way to extract the URL, suggestions are welcome);
2. The `basedir` option is also used in front end,  
    which would make `img(src="/test/image.png")` compile to `<img src="/base/dir/test/image.png"/>`.

Please verify.