Add config for oxford commas

[bdarcus]

Not sure where this should go; Locale (to accompany punctuation_in_quotes) or Style::Config?

[denismaier]

I've always thought that would be a matter of American vs British English, but apparently that does not seem to be entirely the case if AP does not use it. So maybe it's more a stylistic preference? OTOH, you won't use it at all in German (and maybe also in many other languages), so adding it to a setting that affects multiple languages could be a bit problematic.

[bdarcus]

So then put it on Style::Config, with default false?

This came up when I was implementing the list processing.