Preview k3d v3.0.0

In the beginning, k3d was a learning project and grew organically without a clear plan in mind.
This and the eagerness to implement all the requested features as fast as possible, resulted in quite a lot of spaghetti code which became a bit more difficult to maintain and extend.

k3d v3 aims to make a few things better:

• a better project structure with more clearly defined module/package boundaries
• cleaner/leaner code
• new CLI syntax to fit better into the Kubernetes ecosystem (syntax similar to e.g. kubectl)
• usable as a Go module from within other Go projects.

Additionally it offers some new or modified features compared to v1.x, even though not every single feature of v1.x found its way into v3 yet.

This is quite an early release, but I'd like to gather feedback rather earlier than later now.
So please test this release, play around with it, open issues and/or pull-requests and let me know, what you think of it via Slack :)