Pypyrslack

- Breaking change. Henceforth, use `pip install pypyrslack`, not `pip install pypyr-slack`

- use new pypyr error types for context validation errors
- unit tests
- version bump

- pypyr cli now invokes as `pypyr pipe` rather than `pypyr --name pipe`
- fix some README grammar
- bump dev status to beta, like pypyr core

no end-user impact