Add style/type to ListFormatter public API

C API for ListFormatter doesn't have support for style/type of the formatter.

The unstable API has `style` parameter which can be "standard", "duration", or "duration-short".

I assume that the selection has been based on CLDR categories that we recently updated ( http://unicode.org/cldr/trac/ticket/9361 ).

We now have two types of lists - standard and unit (and we're talking about adding a third for formatting list of numbers)

And we have three styles of lists - long, short and narrow.

It would be great to get the stable API to support those two parameters.