@scribu how should we handle synopses that allow passing arbitrary associative arguments?

Right now running the behat test via `php behat.phar features/db.feature` results in:

```
    When I run `wp db export /tmp/wp-cli-behat.sql`                   # features/steps/basic_steps.php:101
      $ wp db export /tmp/wp-cli-behat.sql
      Success: Exported to /tmp/wp-cli-behat.sql

      Warning: The `wp db export` command has an invalid synopsis part: ...
      cwd: /tmp/wp-cli-test-run-5251bfd4bfed58.38985113
      exit status: 0
```