Enable PrivateNamingConvention scss-lint rule

Since we mention that prefixing names with underscores is an acceptable
way to denote that they are private, we want to enforce that they are
defined within the same file that they are used. The new
PrivateNamingConvention rule in scss-lint is designed for this purpose.

Author: lencioni

.scss-lint.yml

@@ -21,6 +21,10 @@ linters:
   NameFormat:
     enabled: true
 
+  PrivateNamingConvention:
+    enabled: true
+    prefix: _
+
   PropertySortOrder:
     enabled: false