Add format argument to parse_*

Add format argument to parse_*

``parse_date()`` does not allow the user to specify the date's format
and the 'medium' format is used by default in the call to
``get_date_format()`` on line 1144.

This results in a failure to parse the date in short format for the
locale 'sv_SE'. This commit adds the format argument to avoid this
failure. The default value is set to 'medium' to preserve the old behavior.

Fixes #657

Co-authored-by: Aarni Koskela <akx@iki.fi>