Enable the ability to pass in a custom codename

While testing out Aptly, the `apt-get` client complains with the following error, since the `codename` was switched from the InRelease files that are baked out by Aptly:

```
E: Repository 'http://debianrepo.example.com/bionic testing InRelease' changed its 'Codename' value from '' to 'testing'
```

completion.d/_aptly

@@ -457,6 +457,7 @@ local keyring="*-keyring=[gpg keyring to use when verifying Release file (could
                             "-distribution=[distribution name to publish]:distribution:($dists)"
                             "-label=[label to publish]:label: "
                             "-suite=[suite to publish]:suite: "
+                            "-codename=[codename to publish]:codename: "
                             "-notautomatic=[set value for NotAutomatic field]:notautomatic: "
                             "-origin=[origin name to publish]:origin: "
                             ${components_options[@]}