Add tutorial on configuration merging

This tutorial was written to provide documentation for zendframework/zf2#4505 -- but was extended to cover the various ways you can manipulate configuration, including:

- environment-specific loading of modules or setting of module-manager flags
- environment-specific application configuration
- order of configuration merging
- manipulating/post-processing the merged configuration (per zendframework/zf2#4505)

This feature was the basis for #904 (which is a backport of the work originally done on this branch), and adds the "CONFIG_MERGE" event information.