Throw an error for invalid date format

[waitingcheung]

When an invalid date format is passed to the validator, calling get returns undefined and if it is further passed to the function date_with_format_array the following error occurs.

TypeError: Cannot read property 'length' of undefined
    at Object.date_with_format_array (file:///path/fieldval.js:902:44)

I check if format_array is an array in the function date_with_format_array first to prevent such problem.

[MarcusLongmuir]

Hi @waitingcheung

Thanks for finding this issue! This PR served as a great starting point and made me realise that a date_format_array check would be rather useful to use as validation for the date_with_format_array check you added. I've raised #27 with my implemenation, but I'm happy to discuss other ways of implementing this that might be better for your use case.

Happy to merge yours first onto develop and then mine on top to give you the deserved contributions.

[waitingcheung]

@MarcusLongmuir The changes look good to me. You may proceed with the merging.