Add anchor option

Add the ability to specify where the datepicker is anchored. 

Default is how it currently behaves, "left". The left border of the datepicker is anchored to the left side of where it is placed.

Specifying data-anchor="right" or specifying {anchor: "right"} in javascript will anchor the datepicker to the right of the element it is placed. The right border will be aligned with the right side of the element.