Add an option to configure how the number of days is calculated

Suppose we have the range 15.06.2016 23:59 ~ 16.06.2016 00:00. The length of time between the start and the end is only 1 second, however this library displays “2 days” because it spans across 2 separate calendar dates. I suggest adding a way to configure this.

Configuration option: daysMode(‘span’ / ‘nights’ / ‘period’)

if (daysMode === 'span') {
    // Use the current behavior (default)
}
if (daysMode === 'nights') {
    // Should be the same as the current behavior minus 1. This is the number of
    // times the boundary between 23:59 and 00:00 is crossed
}
if (daysMode === 'period') {
    // The actual number of complete 24 hour periods between one datetime and 
    // another.
    // If we have Monday at 23:59 until Tuesday at 23:58 this should be 0, even 
    // though it crosses from one day to another.
}

I haven’t looked at the code, but I can’t see this being overly difficult to achieve. If you are interested let me know and I’ll code it and submit a pull request.

[multiwebinc]

@holtkamp For example in the “Typical usage, hotel booking” example on http://longbill.github.io/jquery-date-range-picker/, you typically select the check in and check out dates, so if I check in on the 3rd and check out the 4th I would expect it to say 1 instead of 2. I can edit the locale to say “nights” instead of “days”. Having it display “2 days” to me is confusing. Anyway, this is how I’m currently doing it:

hoveringTooltip: function(days) {
    nights = days-1;
    if (nights === 0) {
        return "Select check out";
    }
    if (nights === 1) {
        return '1 night';
    }
    return nights + ' nights';

As for the "period" example, I can’t really think of a use case, but if we’re going to allow it to be configured, we might as well as there could be somebody who wants to do it like that.

[multiwebinc]

@barryvdh Could be as easy as setting a custom language, like https://github.com/longbill/jquery-date-range-picker/blob/31d1eafe296816b7639fd2e91e8ca23c8775e187/demo.js.