chore(lint): add custom sass linter to require !default

.scss-lint.yml

@@ -2,6 +2,8 @@
 
 # See config at https://github.com/brigade/scss-lint/blob/master/config/default.yml
 
+plugin_directories: ['.scss-linters']
+
 exclude:
   - 'src/components/item/item.ios.scss'
   - 'src/components/item/item.md.scss'
@@ -19,6 +21,9 @@ linters:
   ColorVariable:
     enabled: false
 
+  DefaultRule:
+    enabled: false
+
   DuplicateProperty:
     enabled: false
 

.scss-linters/default_rule.rb

@@ -0,0 +1,12 @@
+module SCSSLint
+  # Reports the use of !important in properties.
+  class Linter::DefaultRule < Linter
+    include LinterRegistry
+
+    def visit_variable(node)
+      return if source_from_range(node.source_range).include?('!default')
+
+      add_lint(node, '!default should be used')
+    end
+  end
+end