Only redact the non-sensitive bits of build args and env vars.

* `-e [REDACTED]` → `-e SOME_SECRET=[REDACTED]`
* Replaces `Utils.redact` with `Utils.sensitive` to clarify that we're
  indicating redactability, not actually performing redaction.
* Redacts from YAML output, including `mrsk config` (fixes #96)

lib/mrsk/cli/main.rb

@@ -119,7 +119,7 @@ class Mrsk::Cli::Main < Mrsk::Cli::Base
   desc "config", "Show combined config (including secrets!)"
   def config
     run_locally do
-      puts MRSK.config.to_h.to_yaml
+      puts Mrsk::Utils.redacted(MRSK.config.to_h).to_yaml
     end
   end