support for showSelectedDays option in custom range

[kavvya97]

Fix for #2364

Added a new optional feature option showSelectedDays for Custom Range option. This feature is by default hidden.
The label for this feature is customizable through locale options.

Note: I have disabled this feature when timepicker is enabled. this is to ensure that we don't want to show the days in fractional value. However, I can add this option if desired.

[Wernfried]

I added this feature in my fork at https://github.com/Wernfried/daterangepicker

You can define the format more granular with parameter locale.durationFormat

Format a custom label for selected duration, for example '5 Days, 12 Hours'.
Define the format either as string, see Duration.toFormat - Format Tokens or an object according to Intl.NumberFormat, see Duration.toHuamn.