Environment names can be used in a number of contexts, and should be
properly escaped for safety. Since most state names are store in path
structures, and often in a URL, use `url.PathEscape` to check for
disallowed characters
Add Env and SetEnv methods to command.Meta to retrieve the current
environment name inside any command.
Make sure all calls to Backend.State contain an environment name, and
make the package compile against the update backend package.
In order to operate in parity with other commands, the env command
should take a path argument to locate the configuration.
This however introduces the issue of a possible name conflict between a
path and subcommand, or printing an incorrect current environment for
the bare `env` command. In favor of simplicity this removes the current
env output and only prints usage when no subcommand is provided.
James Bardin