Adds `acceptedFormats` option

This commit adds an `acceptedFormats` option that represents an array of
accepted input formats. On input change if you're using moment.js this
array will then be passed instead of the format string. This allows you to
accept multiple formats manually, but still get a consistent output and
display format.

See http://momentjs.com/docs/#/parsing/string-formats/ for the moment.js
functionality.