Added support for custom-location to PNG fallback

Using a custom XLink attribute named `xlink:hrefalt`. The attribute name is unofficial and could be something else [better]; "hrefalt" was chosen based on `hreflang`.

This commit also applies this pull request : https://github.com/yoko/svg4everybody/commit/ef2f705c24935851f5bba24b658a7f8efdfb0f63