Add "ps" and "inject" commands

This adds two new commands to pickett.  The first is `ps` similar to `docker ps`.  It's most useful feature is printing the docker container ID.  The second is `inject` which executes a command inside an existing pickett topology node.  Both are quite hacky code-wise, but they move things forward on making pickett more usable for development.
NB:

```
cont = strings.TrimPrefix(cont, "/")
```