v4: option extraFormats to improve parsing

[Koloto]

moment supports an array of formats to parse. I think it's a good idea to support several 'parse' formats and one 'output' format in the datetimepicker too. So I introduced an option extraFormats - an array of optional formats which can be used while parsing. format option specify output format and it is also used as the last parse format. Why the last? For example I want to display four-digit year but also support two-digit year. I specify the following options:

{
    format: 'DD.MM.YYYY',
    extraFormats: [ 'DD.MM.YY' ]
}

According to the documentation moment "prefers formats earlier in the array than later":

moment('1.1.1', [ 'DD.MM.YYYY', 'DD.MM.YY' ]); // 0001-01-01 - not clear
moment('1.1.1', [ 'DD.MM.YY', 'DD.MM.YYYY' ]); // 2001-01-01 - correct

So we must place longer formats at the end of array.
I suppose output format is typically the longest. But if someone wants to change this logic he can add format value into extraFormats at any index, e.g.:

{
    format: 'DD.MM.YYYY',
    extraFormats: [ 'DD.MM.YYYY', 'DD.MM.YY' ]
}

I also looked at #616: there the first element of array is always used as output format. I think it's incorrect in many cases (see example above).

P.S. Sorry for my English:)