Add [-a|--app CODE] parameter to docker:create command

To enable one line installation with the new boilerplate stucture (using "app" instead of "code"-dir), an additional parameter "--app" (beside "--code") is needed.

After adding it, it could be used like this:
`ct docker:create --docker=typo3 --app=git@bitbucket.org:projectname/app.git projectname`
