add top-level `forwards` section

[nickthecook]

Consider a project in which there is:

• a top-level infrastructure project, e.g. terraform code, that creates infrastructure and deploys an app to it
• an app directory, self-contained, with its own ops.yml, that can be run on a development machine or on deployed infrastructure

It's common in this case to have the top-level project provide secrets and config that are also needed by the app project. In that case, the developer must choose one of the following options for using top-level secrets and config in the app:

• duplicate the config and secrets (not great)
• generate app config and secrets in the top-level project (it's a pain to generate an ejson file from terraform in all your apps, and makes it harder for a developer to find the config and secrets since they're not checked into the app directory where they're used)
• "forward" a bunch of ops commands to the app dir from the top-level dir (verbose and ugly - see below)

"Forwarding" in ops looks like this:

actions:
  pack:
    command: cd packer && ops pack-all
    alias: p
  pack-force:
    command: cd packer && ops pack-force-all
    alias: pf
  pack-all:
    command: cd packer && ops pack-all
    alias: pa
  pack-force-all:
    command: cd packer && ops pack-force-all
    alias: pfa

But fowarding could look like this:

forwards:
  packer:
    - pack, p
    - pack-force, pf
    - pack-all, pa
    - pack-force-all, pfa

This is more idiomatic, communicating more clearly to the reader that a subdirectory handles these. It's like the difference between a for loop and the ruby .each: obvious intent (and less typing).

In the case of having subprojects inherit top-level secrets and config, however, it doesn't work when running on deployed infrastructure. The top-level dir shouldn't be copied to the infrastructure at all. Is there another use case where this does make sense?

[nickthecook]

Even better, forward an action like app to the app directory, so, e.g.:

forwards:
  app: app

would result in ops app start running ops start in app/. ops app stop would run ops stop in app/. No need to forward each action, but the user will still know where to look to find a command that is being run.

[nickthecook]

Fixed in 0.19.0, as comment described it.