Prepare Wind Tunnel framework to be used with hApps

[ThetaSinner]

We'd like for people to be able to write their own Wind Tunnel scenarios. To enable that, we need to lay some ground work.

AC:

• A versioning strategy for Wind Tunnel is defined such that we can release versions of it for multiple versions of Holochain while also evolving the framework. This must be documented.
• A guide has been written to describe how Wind Tunnel scenarios can be created in a hApp project.
• There is documentation describing how to execute scenarios across multiple test nodes. Could this be done by documenting our own process or should we recommend something simpler?
• The terminal reporter is extended to report custom metrics so that users can get local results without running InfluxDB
• Decide whether to release the library used in the summarizer. That would need some refactoring so that it's not specific to the Holochain scenarios in this repository but parts of it are re-usable.
• There are regular test runs to check that Wind Tunnel releases work with new versions of Holochain that are expected to be compatible. E.g. On the latest version of Wind Tunnel for Holochain 0.4.X, 0.4.X + 1 is released and we check that Wind Tunnel can still run against it.
• There are regular releases for Wind Tunnel when Holochain requires changes to it, so that users can move their tests forwards to test against the latest versions of Holochain.

[ThetaSinner]

Need spike results from #111 before refining

and we're going to delay this until after the MVP epic is done