Date validator request

This package is currently not providing a Date validator, which would be very useful for validating date-type inputs. Such a validator is already provided by `ember-validators` but is not in this project.

https://github.com/offirgolan/ember-validators/blob/v2.0.0/addon/date.js

Having it available would allow us to use changesets for date inputs with something better than
```javascript
validateFormat({ regex: /[0-9][0-9][0-9][0-9]-[0-9][0-9]-[0-9][0-9]/ })
```

which is currently the best, but somewhat hacky, implementation option.