Access custom set months in captionElement

[grazinskis]

Seems like inside captionElement there is no way to access the months that are set with the 'months' prop on DayPicker component.

I replicated the issue here in example.js line 69.

My use case is that I set my translated names manually on DayPicker, and then I would like to keep those names as the single source of truth. Now I have to set them in my captionElement for the second time. I think a good solution would be to expand the captionElement props to expose custom set months and weekday names.