prop to set only start date, or only end date

[gsouf]

Subject of the issue

I'd like to have an option to set only the start date, or only the end date when selecting.

I tried retainEndDateOnFirstSelection that is almost what I want but it wants to select a second date after the first click. I want that a single click chagnes the start date and that's all. I'm also looking for the same prop for only changing the end date.

Environment

Package Version: 1.4.0

I'm happy to provide a PR for this feature.