Validate "value" property passed to sl-date-picker

[azizpunjani]

{{sl-date-picker value="11111"}}

Currently, the above would pre fill the date picker with 11111. The value should be parsed to make sure it's a valid date before being passed to the input.

[SpikedKira]

#1458 will likely close this issue.

Previously the date was bound to the 'value' property, which is a string. #1458 changes this to force binding on an object of type moment, using the 'selectedDate' property. The value property of the input tag is now generated from 'selectedDate' and parsed by property 'format'.