If a Drush command returns a string value, then print it

... even if the command does not define an outputformat record in its command definition.

Previously, a command needed to define an outputformat record in order to get the string to print. e.g.:
```
'outputformat' => array(
  'default' => 'string',
),
```
With this patch, this definition may be omitted; however, omitting the outputformat record is not quite the same as including a simple 'string' definition, as shown above.  If the outputformat record is included, then the help text for the command will include the option for --format, and it will be possible to format using universal formatters such as var-export and json.  Without the outputformat record, the --format option will not appear in the help text, and --format cannot be used with the command.

For simple commands that output only strings, it is probably most appropriate to omit the outputformat record, to keep the command help definition simple.