Allow for Pagination to be performed on sub page

Added new boolean config variable - "sub_page". When this is true, the $url variable is set to the "page_indicator" variable.

Example usage:
1. Create a page called "blog" with a new template that lists the pages with dates (as per usual pagination)
2. Set "page_indicator" to "blog"
3. Set "sub_page" to true
4. Have landing page as main site index, and paginated list of posts under /blog - allows for different template to be used for each, and means the pagination does not have to be on the root of the site.
Added information about the "sub_page" variable
Added preg_match condition for pagination numbers
Added explanation of how to us sub_page