Make greater interval units and weeks work, add more known aliases

* `millennium`, `millennia`, `millenniums`, `mil`, `mils` for a thousand years
* `century`, `centuries`, `cent`, `c` for a hundred years
* `decade`, `decades`, `dec`, `decs` for ten years
* `yr`, `yrs`, `y` shortcuts for years
* `week`, `weeks`, `w`
* also `d` for days