Support multiple nesting levels in Roda#assets

The existing code only worked up to a single nesting level, so you
could do :css=>'foo' (unnested) and :css=>{'foo'=>'bar'} (1 level),
but not :css=>{'foo'=>{'bar'=>'baz'}}.  However, the routing
supports multiple nesting levels, so it doesn't seem wise to have
the rendering restrict it.

There may be other parts of don't work with multiple nesting levels,
those should be fixed.

This removes another . to $2E conversion.