message: add emptyLabel prop to docs
diff: @@ -97,7 +97,8 @@ minDate | date | '1900-01-01' | Minimum selectable date
  maxDate | date | '2100-01-01' | Maximum selectable date
  onChange | func | required | Callback firing when date accepted
  returnMoment | boolean | true | Will return moment object in onChange
  -invalidLabel | string | 'Unknown' | Displayed string if date cant be parsed (or null)
  +invalidLabel | string | 'Unknown' | Displayed string if date cant be parsed
  +emptyLabel | string | '' | Displayed string if date is `null` (e.g. after clear)
  okLabel | string | 'OK' | The label for the ok button
  cancelLabel | string | 'Cancel' | The label for the cancel button
  clearLabel | string | 'Clear' | The label for the clear button
  @@ -119,7 +120,8 @@ format | string | 'MMMM Do' | Moment format string for input
  autoOk | boolean | false | Auto accept time on selection
  onChange | func | required | Callback firing when date accepted
  returnMoment | boolean | true | Will return moment object in onChange
  -invalidLabel | string | 'Unknown' | Displayed string if date cant be parsed (or null)
  +invalidLabel | string | 'Unknown' | Displayed string if date cant be parsed
  +emptyLabel | string | '' | Displayed string if date is `null` (e.g. after clear)
  okLabel | string | 'OK' | The label for the ok button
  cancelLabel | string | 'Cancel' | The label for the cancel button
  clearLabel | string | 'Clear' | The label for the clear button
  @@ -146,7 +148,8 @@ minDate | date | '1900-01-01' | Minimum selectable date
  maxDate | date | '2100-01-01' | Maximum selectable date
  onChange | func | required | Callback firing when date accepted
  returnMoment | boolean | true | Will return moment object in onChangeg
  -invalidLabel | string | 'Unknown' | Displayed string if date cant be parsed (or null)
  +invalidLabel | string | 'Unknown' | Displayed string if date cant be parsed
  +emptyLabel | string | '' | Displayed string if date is `null` (e.g. after clear)
  okLabel | string | 'OK' | The label for the ok button
  cancelLabel | string | 'Cancel' | The label for the cancel button
  clearLabel | string | 'Clear' | The label for the clear button
  