Exec for some containers

This PR adds two closely related features:
* the ability to apply a command against a subset of the containers specified in the configuration using the new cmdline option `--target`.
* the ability to define containers in your config/manifest that will not be automatically started by the run or start commands.  They will only be started if specified using the cmdline option `--target`.

Sometimes one might want to restart a single container.  Sometimes one may want to define a container in the same manifest file, but only have it run when specifically wanted.  An example is if the manifest is setting up a Django or Rails server but sometimes you want to run `manage.py` or `rake` tasks in a similar environment.  Now, you can define the container for those tasks but not have them automatically launched at the same time as your appserver and webserver.