Adds a set of commands to work with and manipulate environments:

• tk env add: Create a new environment, optionally setting parameters (now also used by tk init)
• tk env remove (tk env rm): Remove an environment
• tk env list (tk env ls): List environments (optionally as JSON)
• tk env set: set properties on environments (the same available for tk env add)

These commands aim to be ks env compatible. However, Tanka infers the environments name from the directory, so it is not possible to modify it using add / set --name. Just mv the directory in that case (included in the error message).

Fixes #72