current-non-scribble-entity-handler

Currently the markdown parser converts "..." (and friends) to an `'hellip` symbol. This works lovely for `xexpr->html`, but `'hellip` isn't a supported `pre-content` symbol.

The parameter: `current-non-scribble-entity-handler` is called when `'hellip` is generated, and is a hook to substitute other "representations" of "...". It defaults to `values`, which leaves the output as the current parser does. It could, equally be:
```racket
(match-lambda ['hellip "..."]) ; literally three dot string
(match-lambda ['hellip "…"]) ; U+2606 - HORIZONTAL ELLIPSIS = three dot leader
```
this handler can be used to extend any other "smart" symbol mappings.