Feature request: Support custom masks

[Ken Kinder]

Right now, you can display a date in any format you want, but you can't have a custom date format and use a masked input to let the user type that format in.

Such functionality would be quite useful. So for example:

<q-datetime-picker
  v-model="mydate"
  input-mask="YYYY-mm-dd"
  />

(Or similar)