Some Ducktype Development

I recently changed the parser directive syntax in the ducktype parser to
resemble the syntax for info elements. Parser directives are things you
put at the very top for things like namespace declarations and entity
definitions. In many cases, you will never use them.

Old Syntax:
[[ducktype/1.0 encoding=utf-8]]
[[ns if="http://projectmallard.org/if/1.0/"]]
[[def foo="some replacement text"]]

New Syntax:
@ducktype/1.0
@encoding utf-8
@namespace if http://projectmallard.org/if/1.0/
@define foo "some replacement text"

The nice thing is that you no longer have to close anything off.

I'm also working on comments, using [[-- --]] or possibly [[# #]]
(opinions or other suggestions welcome). These would be parsed at the
line level only, not e.g. inline. So a starting [[-- has to be at the
beginning of a line, possibly preceded with whitespace.

Thoughts?

--
Shaun