added process complete hook

use case I want to show a message when there are errors and when there
are not.

```
  var noErrors = true;
  gulp.src('.')
      .pipe(enforcer({ … }))
      .on('error', function(e) {
        noErrors = false;
        // log errors to the console
      })
      .on('end', function(e) {
        if (noErrors) {
          // log coverage levels ok to the console
        }
      });
```