Consider adding disablable preparser

[twitwi]

only if preparsers are actually chainable (like the key bindings)

include by default the preparser evaluator from hdr slides (___n=1___) [___n++___] (and maybe also a multiline version of (___ to be able to define function more easily (___o.title = n => '## Section '+o.current+'.'+n___) ! ... also allow [___ to return a list of lines.)

maybe also a few magic entities like &| &0 ... or is MDC sufficient for those?

and update doc...