Add a new `date` field that abstracts over `day`, `month`, and `year` and allows date queries

In some cases it would be useful to use [date and date range querys](http://beets.readthedocs.io/en/v1.4.6/reference/query.html#date-and-date-range-queries) with the `year` tag.

So a query like:
`beet list 'added:-5y..'`
would also be useful with the year tag:
`beet list 'year:-5y..'`
This query would list all tracks from the last 5 years.
It is more dynamic than those:
```
beet list 'year:2013..2018'
beet list 'year:2013..'
```

Any thoughts on that?