Add a @local decorator

[romain-intel]

Currently you can use @batch to decorate single steps that you want to force to run on batch even without using --with batch. However, you cannot do the opposite and force steps to run locally even when doing --with batch.

A possible use-case for this would be to annotate a GPU-heavy step that you want to run locally to save on AWS costs but take advantage of fast S3 access for other steps (from the EC2 machines batch uses).

It is currently possible to have this use case by annotating every other step with @batch but this is not very developer friendly (you can't easily run your code locally to test, it's tedious, etc).

Relevant discussion on gitter:
https://gitter.im/metaflow_org/community?at=5f85f41d24a20801a8e5373d

Other questions to address when touching decorators:
https://gitter.im/metaflow_org/community?at=5f7f81e024a20801a8d509a6
https://gitter.im/metaflow_org/community?at=5f05d3baa5ab931e4f7280e3

[romain-intel]

This one should be simple and useful.