text: Title: can't make selection for DatePickerRange when using placeholder_text
  Body: no selection option will appear when using `start_date_placeholder_text` or `end_date_placeholder_text` with `dcc.DatePickerRange`
  
  See doc page for example: [https://dash.plot.ly/dash-core-components/datepickerrange](https://dash.plot.ly/dash-core-components/datepickerrange)