Delegate date formatting

[snovakovic]

Delegate date formatting. This will allow us to use moment, date-fns, globalize or any other library to format dates.

This address issue #211

Usage:

<datepicker :format="formatter"></datepicker>

methods: {
  formatter(date) {
    return moment(date).format('MMMM Do YYYY, h:mm:ss a');
  }
}

[charliekassel]

Available from v0.9.4