Stateful commands

I have a suggestion: instead of typing something like

```
aws> ec2 describe-instances
aws> ec2 run-instances --blahblah blah
```

you could enter the prefix once, and all subsequent commands are assumed to include that prefix:

```
aws> ec2
aws ec2> describe-instances
aws ec2> run-instances --blahblah blah
```

then use `exit` or similar to move to the upper level.

Start by tracing the interactive command loop and command parsing used by aws-shell. Compare the requested nested prompt, prefix handling, and exit behavior with the current shell behavior. Done means users can enter a command prefix, run subsequent commands within that context, and exit back to the upper level.

Written by the indexing model from the issue text.