Render noExpander icon from icon map for leaf nodes

I'm making this PR to highlight a feature request (no expander icons on leaf nodes), and to propose one limited solution for my specific use-case.

This is not the right answer and **should not be merged**: it works for FontAwesome where the .fa class is a good way to get an empty space. I don't know whether glyphicons has anything similar. And this patch doesn't solve the problem for non-glyph rendering where an empty image is probably required.

For reference, this patch requires this as part of a fontawesome config for the glyph extension: ```glyph:{map:{noExpander:"fa"}}```