[multiselect] Add a function to get preMultipleDates

[nyroDev]

As you can see in #209, there is some use case when we need the preMultipleDates.

I could be great if we could have an utility function to get these dates mapped as DateTime, exactly how getMultipleDates does. It will be especcially useful in multiselect.select event, when other UI has to be updated regarding these dates.

As an extension, I think it could also be useful to have a clearPreMultipleDates and preMultipleDatesToString