Added [[include:]] syntax to allow limited nesting of other pages.

Is this a reasonable feature? You can add [[include:Link]] to render one page within another (currently hardwired to do this 10 deep before rendering an error). The link syntax is the same as for normal links. I added a common function for rendering errors based on what was there already - it will render an error if the page doesn't exist in the wiki, or after 10 nested includes.