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
 

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

lib/mrsk/commands/registry.rb

@@ -2,7 +2,7 @@ class Mrsk::Commands::Registry < Mrsk::Commands::Base
   delegate :registry, to: :config
 
   def login
-    docker :login, registry["server"], "-u", redact(lookup("username")), "-p", redact(lookup("password"))
+    docker :login, registry["server"], "-u", sensitive(lookup("username")), "-p", sensitive(lookup("password"))
   end
 
   def logout