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/commands/builder/base.rb

@@ -28,7 +28,7 @@ class Mrsk::Commands::Builder::Base < Mrsk::Commands::Base
     end
 
     def build_args
-      argumentize "--build-arg", args, redacted: true
+      argumentize "--build-arg", args, sensitive: true
     end
 
     def build_secrets