Add "before" and "after" date parameters

### Description

It isn't possible to pass a date query via a flag argument (see also https://github.com/wp-cli/wp-cli/issues/4616), but we could accept `--start-date` and `--end-date` flags that map to a date query.

### Use Case

We were once asked to provide an audit for posts published in the last 12 months. We had to settle for two year-based queries.