Added WPCLI method to "exercise" a site

The resulting exercise can then be reported in New Relic APM for better performance profiling.
Since this isn't an object accessor function, I think it would make sense to invert this if statement and move the brunt of the work outside of the control structure:

```php
if ( $this->wpcli )
{
  return TRUE;
}//end if

require_once __DIR__ . '/class-go-newrelic-wpcli.php';

// declare the class to WP:CLI
WP_CLI::add_command( 'go-newrelic', 'GO_NewRelic_Wpcli' );

$this->wpcli = TRUE;
```