Add new management commands for siteconfig.

This adds some useful management commands for listing the current
site configuration, setting an existing key, or displaying the value of
a key. It should help when correcting configuration or debugging a
configuration problem.

The new commands are:

    * list-siteconfig
    * get-siteconfig
    * set-siteconfig

Tested list-siteconfig, and saw the whole site configuration.

Tested set-siteconfig with valid and invalid keys of different depths.
Tested setting strings, integers, and booleans.
Tried overriding arrays and dicts, and got the proper error.

Tested get-siteconfig with valid and invalid keys, and different
value types.