Application developer might want to include additional classes for as bootstrappers

This give them an easier way to do that without overriding the default options. E.g:

    protected function getBootstrappers()
    {
         return parent::getBootstrapper() + ['FooBoostrap']:
    }

Signed-off-by: crynobone <crynobone@gmail.com>
Merge pull request #6347 from crynobone/feature/configurable-bootstrapper

[5.0] Application developer might want to include additional classes for as bootstrappers