Adds test for empty configuration files. Refs #21.
Removes docgen baggage from default configurations. Refs #21.
Adds layered config to always have default values. Refs #21.
Fixes API breakage due to dual config. Refs #21.
Moves enums to new lconfig file. Refs #21.
Adds constant key to access configuration value. Refs #21.
Moves split.item_toc to lconfig submodule. Refs #21.
Moves more constants to lconfig submodule. Refs #21.

Also renames some to have a more consistent naming.
Moves to lconfig special failure config key. Refs #21.
Hyperlinks render keys from error documentation. Refs #21.
Extracts more configuration constants to lconfig. Refs #21.
Tests custom date/time formatting strings. Refs #21.
Renames split.item.toc for consistency. Refs #21.
Documents task in changes log. Refs #21.
Merge branch 'feature/t21_additive_config'