(cli) Feature Request: A way to cancel and watch running updates

## :rocket: Feature Request

### General Information
* [X] :wave: I may be able to implement this feature request
* [ ] :warning: This feature might incur a breaking change

### Description
I'm testing ECS deployments using CDK, and I hit the timeout issue described at aws/aws-cdk-cli#1433.

I'd like a way to cancel a running stack update from within CDK rather than needing to use the console or aws-cli in another terminal.

I'd also like to be able to "watch" an ongoing update on a stack from another invocation of CDK - eg, if my AWS session token timed out, or I switched machines, etc.

### Proposed Solution

Add two new subcommands:

`cdk cancel `: If this stack is currently in `UPDATE_IN_PROGRESS` state, send a `CancelUpdateStack` message, then exit.
`cdk watch `: If this stack is being updated or rolled back, show the log up until now, then continue to tail the log (as if you'd run `cdk deploy`)