lua: Add functions to register some missing callbacks

lua: Add functions to register some missing callbacks

* register_init
* register_shutdown
* register_config

In this version, the configuration for each Script is
supported.

  Script "foo.lua"
  Script "bar.lua"
  <Module "foo.lua">
    Key1 Value1
  </Module>
  <Module "bar.lua">
    Key2 Value2
  </Module>

The scope of accessible key-value pair is limited to under <Module>
This feature is similar to collectd-python.

  ref. https://www.collectd.org/documentation/manpages/collectd-python.html

Signed-off-by: Kentaro Hayashi <hayashi@clear-code.com>