Allow to change isExpanded dynamically + Support null safety

[zvikarp]

Hi, loving this package 😍!

In my app I need to change the expanded state with a custom button, to do so I made minor changes in the package code. I'm opening this request to merge the changes, I have tested it ant all works! You can change the value of isExpanded from the parent widget and it changes the calendar expanded state.

Also noticed that an issue on this subject was opened a few weeks ago, so this PR can close #31 .

[zvikarp]

I noticed that hideBottomBar is not in use, instead, we use isExpandable:

https://github.com/pmcarlos/flutter_clean_calendar/blob/master/lib/flutter_clean_calendar.dart#L272

I'm not changing it, as users might be relying on it, but it is something to notice, maybe for the next major version.

[zvikarp]

Hi, A long time has passed since this PR was opened, I have also added null-safety to my fork.
This Pr closes #35 and doesn't use the date_util package anymore (it is implemented in this package), as it doesn't support null-safety yet.

[pmcarlos]

Thanks for this @zvikarp, and sorry for the delay