Feature request: Add support for variables

Not sure about the specifics, but at a high level, it would be nice to do something like:

```
aws> variable=$(ec2 describe-instances --query "Reservations[0].Instances[0].InstanceId" --output text)
aws> ec2 start-instances --instance-ids $variable
```

No files, tests, or entry points are named. Start by tracing how the shell parses and executes commands, then define how assignment and variable substitution should work for the shown AWS CLI example; done means both commands execute with the captured instance ID.

Written by the indexing model from the issue text.