parse.rkt: added current-non-scribble-entity-handler

Scribble does not include 'hellip as a recognised HTML entity in pre-content

Entities not in: 'mdash, 'ndash, 'ldquo, 'lsquo, 'rdquo, 'rsquo, 'larr, 'rarr, or 'prime are passed through current-non-scribble-entity-handler

current-non-scribble-entity-handler defaults to `values` i.e. `(current-non-scribble-entity-handler 'hellip)` -> `'hellip`
other possibles are:
```racket
(match-lambda ['hellip "..."]) ; literal dot dot dot replacement
(match-lambda ['hellip "…"]) ; unicode horizontal ellipsis character
```