Optional repositioning [#79740240]

Billable to B+M.

Adds an option, `reposition`, which defaults to `true` to leave current behavior.

When set to `false`, it will allow the datepicker to be positioned outside of the current viewport.

The use case is that we need the datepicker to always appear below the input regardless of the input's position on the page so that it does not cover up the label and help text as users are getting confused.

Also PR'd to original here: https://github.com/dbushell/Pikaday/pull/192