[multiselect] multipleDatesToString could use the global format option by default

[nyroDev]

Is your feature request related to a problem? Please describe. When using the multipleDatesToString, the format used as to be set in the call.
We already set a format option in the global LitePicker option.

Describe the solution you'd like If the format option is omited (false/null) in the multipleDatesToString call, then use the global format option. The signature of multipleDatesToString should probably change to have the sep option first, and then the format. I will allow us to simply call picker.multipleDatesToString(', '); and not picker.multipleDatesToString(false, ', '); as requested here. It will be a breaking change, I don't know how it's handled here.