No redaction for with values

Since it's common for tools to use credentials from env variables rather than inline in commands it seems generally useful to support redaction for them as well.

I accomplished that with
```ruby
def with_redaction
new_args = args.map{|arg| arg.is_a?(Redaction) ? '[REDACTED]' : arg }
redacted_cmd = dup
redacted_cmd.instance_variable_set(:@args, new_args)
if (env = redacted_cmd.options[:env])
redacted_env = env.transform_values { |v| v.is_a?(SSHKit::Redaction) ? '[REDACTED]' : v }
redacted_cmd.instance_variable_set(:@options, redacted_cmd.options.merge(env: redacted_env))
end
redacted_cmd
end
```