Add utility function to execute docker commands and retrieve the stdout without changing the UI

[jakecode]

For some functionality it would be useful to be able to execute non-volatile docker functions with the output being redirect to somewhere that is not the UI, such as a hidden buffer.

Currently any commands ran using container_exec or container_action_run will modify the open terminal window, which mean that the plugin is only as aware of docker objects as the users current view is.

Being able to run these commands and manipulate / interpret the output silently would add a good amount of scripting power by giving access to things like the docker container inspect, or docker stats without having to override the users current view.