Add labels option to calendar range

[AKostiv8]

Short description (with a reference to an issue).
This PR adds users an opportunity to add label(s) about the calendar_range input fields, as for now it is quite complicated to add labels above start and end calendars fields precisely.

calendar_range(
          input_id = "calendar_range",
          start_value = "2020-02-20",
          end_value = "2020-03-20",
          min = "2020-01-01",
          max = "2020-12-01",
          start_placeholder = "Select range start",
          end_placeholder = "Select range end",
          start_label = "Start Date",
          end_label = "End Date"
        )

In general, to existing calendar_range function added two arguments start_label and end_label
The example app showcasing has been also completed. examples/calendar_range

DoD

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[aniaskrzydlo]

I think we don't have a task in the backlog for this change, so please add the task, or if it's there just link it to the PR.
The functionality works well, I've just added two suggestions to change description and the order of arguments.

[aniaskrzydlo]

How about changing the descriptions to 'Label to be displayed with the start calendar' and 'Label to be displayed with the end calendar'?

[aniaskrzydlo]

I've checked that in the majority of inputs we keep the order of arguments the same as in basic shiny inputs, this is: id, label, value/choices. How about putting the labels after the input_id argument?