This adds a new `--records-loose` flag and changes the default behavior
to `--records-strict`

The rationale for this change is so that this default behavior helps
more when users are still experimenting with the appropriate type to
use to decode a large and messy YAML file.

For example, suppose a user is trying to translate the following YAML
to Dhall:

```yaml
foo: 1
bar: true
```

... and they start with a empty Dhall type which they plan to fix in
response to error messages.  They'd get a surprising success:

```bash
$ yaml-to-dhall '{}' < ./example.yaml
{=}
```

... because the old default behavior is to ignore extra fields in the
YAML.  However, if we're strict by default then users don't have to
apply the `--records-strict` flag to get feedback on what they need
to change.