Added FormatDate migrate process plugin

Previously there was no simple way to reformat a date string into the proper format for use by the core date field.

Use it like this:
Example usage for date only fields (DATETIME_DATE_STORAGE_FORMAT):

process:
  field_date:
    plugin: format_date
    from_format: 'm/d/Y'
    to_format: 'Y-m-d'
    source: event_date

Example usage for datetime fields (DATETIME_DATETIME_STORAGE_FORMAT):

process:
  field_time:
    plugin: format_date
    from_format: 'm/d/Y H:i:s'
    to_format: 'Y-m-d\TH:i:s'
    source: event_time