Added openPos property resolve issue 'Set opening direction? #347'

[moriarty83]

This update adds functionality to allow the user to pass an openPos prop of 'left' or 'right' (default of 'left') to determine of the calendar originates from the left or right side of the input element. This should help to resolve issue #347 which can cause the calendar to exceed the boundaries of it's parent div if it is close to right edge of the div/screen.

Specifics:

• Added prop openPos to Datepicker.vue, defaults to 'left'.
• openPos prop is passed to PickerDay.vue, PickerMonth.vue and PickerYear.vue
• openPos is then read into the :class of each component and if 'right' a calss of 'open-right' is added to the component.
• added &&.open-right to the .vdp-datepicker__calendar which sets 'right 0px'.

[Nate603]

seems like a nice feature to have, why no merge xD