Custom display_name handler.

Currently, if we want to tweak how a function name is displayed we have to use our own template. It's unfortunate.

The following patch makes it possible to define a display_name handler in our `config.ld`.

The documentation contains an example that shows how to print an icon/label beside a function marked with a certain tag. (That example uses the `hidden` feature from #121 but it's not mandatory; I could remove it.)