Major: support rootpages (README.md) and custom pagesort based on in-file metadata

- Add rootpages like README.md to the toplevel sidemenu (optionally disable)
- Enable custom pagesort based on custom metadata in the files (optionally disable)
  Also enables the use of other metadata, like Title
- Documentation for both features

Note: Both features also available in separate branches.

See also: https://groups.google.com/forum/#!searchin/silverstripe-dev/martimiz/silverstripe-dev/d8jTpkQhT88/JjwikBCxscwJ