Page as shortcode

Hi,

Currently, SimplePages allows to display shortcodes, but simple pages are not shortcodes themselves, so they can't be included inside simple pages. 

The main purpose of this patch is to build blocks with shortcodes like (`[simple_pages slug=about]`), but it is possible to display a list of simple pages too (`[simple_pages slugs=about,my_page,other_page output=link]`), or to display a hierarchical navigation when needed (`[simple_pages output=navigation]`) (more details in the readme). 

Sincerely,

Daniel Berthereau
Infodoc & Knowledge management