Ducktype directive includes

I've implemented includes for parser directives in Ducktype. Note that
there are a few places we might want to do includes, with varying
degrees of difficulties. This is the easy one.

Directives are lines at the top of the file that provide namespace
prefixes, entity definitions, and other goodies. You can now put all
those into a common file and just include them.

common.txt
----------
@namespace if http://projectmallard.org/if/1.0/
@namespace ui http://projectmallard.org/ui/1.0/
@define myapp $app(My Application)


index.duck
----------
@include common.txt

= Page Header

[note if:test=target:html ui:expanded=true]
  . Note Title

  This note for $myapp; only appears in HTML output.
  It can be expanded and collapsed by clicking the title.


--
Shaun