Fix to allow customization of blog.filters.syntax_highlight.css_dir

It is currently not possible to configure the pygments css files generated by the syntax highlight filter to be written to any directory other than '/css'.

This is because the attribute lookup for `config.css_dir`, the default value for the `location` argument of `write_pygments_css()`, is performed at compile-time, before the user’s `_config.py` has been read.

This fixes that.