Validate indentation of workflows section
Currently CircleCI warns nothing if the config.yml has wrong indentations like this:
In this example, `filters` has a wrong indentation (it must be indented one level deeper) so it has no effect. But sometimes it is hard to find this type of mistakes because CircleCI just ignores them.
It is nice if CircleCi denies such wrong indentations and makes it `Build Error`.