Ability to pass in an ISO "lastmod" string directly

Downstream: NodeBB/NodeBB#1578

Would like to pass in a parsed ISO string into the library, instead of having the library do it for me, so I added a new property, `lastmodISO` (I'd prefer `lastmod` myself, but in the interest of backwards compatibility, I called it `lastmodISO` instead)

Since I store my values in my db as unix timestamp in ms, it seems roundabout to:

1. Convert ms to date object
1. Parse date object into YYYY-MM-DD
1. Pass that into your module and have it parsed back into a Date object
1. Parsed back into YYYY-MM-DD again

So I'd like to just pass it in directly.