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/base.rb

@@ -1,6 +1,6 @@
 module Mrsk::Commands
   class Base
-    delegate :redact, :argumentize, to: Mrsk::Utils
+    delegate :sensitive, :argumentize, to: Mrsk::Utils
 
     attr_accessor :config