Add macro to display page tree with links.
Add macro to display page tree with links.

Pages and Directories are displayed as nested unordered lists, where pages
link to their page and directories link to theirs index page.

Signed-off-by: flozzone <b465f0a6d1e1aa251082c199a74a8496a9a6ef30@gmail.com>

flozzone/gollum-lib,flozzone/gollum-lib