Add --out-dir flag to grow stage and grow deploy

[jeremydw]

We have a use case where we want to run JavaScript tests against the grow build or grow deploy fileset (after the commands are run).

In order to generate a local fileset for the JavaScript tests, we need to run a separate grow build and then use a grow deploy with the --work-dir flag.

It would be nice to avoid having to add that extra step by adding a --out-dir flag to grow build and grow deploy that will write the fileset to a specific path (and leave the files there, rather than deleting them) after the deployment is finished.

Then further work can be done against that fileset external to the stage and deploy commands.

Example:

# Stages files, and also writes them to `foo`
grow stage --out-dir=foo

# Do other stuff, e.g. run a JS unit test against the `foo` files.
npm run test