Improve dox header implementation

[mitchell-then]

The dox header contains information about a page, and how it should be published. The information takes the form of a page ID and source directives. Right now, the directives are simple flags that modify dox publishing behavior.

More directives have been added, exposing the need for a better, scalable solution of a dox header. Some things to keep in mind:

• solution should be scalable, allow for more directives to be cleanly added
• directives may not be boolean flags, allow for directives of different values