Merge with primary schema

[nixjdm]

Added the optional ability to mark schemas for merging. Any marked schema is merged into the default_map, just as the primary schema is. I also added a note about this to the docs and added a test.

Example usage can be seen in the tests. In short, this uses another list in ConfigFileReader called config_section_primary_schemas, similar to config_section_schemas.

Default behavior is unchanged.

This is a solution to #2

[kdeldycke]

I'd love to see that PR merged! :)

[gilesknap]

Yes, this looks like a good feature to have!