RFC: empire build run ps subcommands

The empire command currently has one subcommand; server. This subcommand runs a full "Empire" environment in server/client mode, backed by a postgres database. However, there's a number of features internally to Empire, that would be incredibly useful outside of a server/client model, in a more UNIXy and composable fashion.

At the core of Empire, it simply takes a Docker image, and produces a "manifest" describing the processes to run, It does this by inspecting the Docker image, extracting a Procfile, and performing mutations on it before handing it to a "backend" to actually build the manifest (CloudFormation).

empire build <image>

Builds a CloudFormation stack for a Docker image.

empire run <image> command

Runs a one-off command (same as emp run).

empire ps <image>

Lists the running tasks for the Docker image.

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Having commands like this could provide a lot of flexibility to compose Empire's internals with existing tools.