Walkthrough demonstrating weave 1.0 features

@rade says:

One way to elucidate all the features we'd like to be part of Weave 1.0, is to imagine what a demo transcript would look like.

Headings denote the new features we are demonstrating in that part of the transcript.

The demo highlights features with minimal effort; for a really compelling demo it may be advisable to re-cast everything in terms of an imaginary system somebody is putting together.

We focus on new features. All these are additive, i.e. all existing functionality is still present but may require slightly different CLI commands than present. Nothing should rely on new features or changes to Docker.

Key theme: ultimate simplicity for the common case
If you got the name wrong then just stop your container and restart it with the correct name.

Yes, this does mean we need to gc 'empty' subnets. But we need that anyway. Can we reap them eagerly, or is there a reason for subnets to survive beyond the last container running in them? Various failure/partition scenarios might be a reason.