Use `dateRange` formatter instead of `date`
Fall back to a dateRange format if provided an array of dates to format

If a date formatter is provided with an array of dates, then output a date range.

Allows accidental mis-configuring of formatting options for date ranges, which now would output "Invalid date".
Change 'dateRange' format to 'date' in stub
Use 'date' formatter instead of 'dateRange'
Get rid of dateRange date formatter

The date formatter now handles both cases.
Merge pull request #756 from alphagov/bugfix/date-ranges

Bugfix/date ranges