Add a metadata page for inspecting configuration.

This is another metadata page that uses the new configuration system to
show all the available options, their current values, and their defaults.
This is still unstyled, but the info is there. I still need to add on a
method for extensions to register their configuration with the global
config so they show up (stuff like blog) but this is a start.
Merge pull request #776 from bhollis/config_meta

Add a metadata page for inspecting configuration.