This avoids harcoding test-specific overrides, and makes it easy to
setup a settings dictionary based on DEFAULT_CONFIG for testing.
Because you can trust Pelican to use settings based on DEFAULT_CONFIG,
you are free to go about using:
settings[my_key]
instead of:
settings.get(my_key, some_fallback)
or:
if my_key in settings:
...
if you know that `my_key` is in DEFAULT_CONFIG.
This dictionary is accessed by plugins (like `summary`) which add new
settings, so it should be public (i.e. no prefixed underscore).
The changed name length would have led to a re-indenting of the
default contents anyway, so I shifted them all to four spaces.
