Fix style picker in markup modal (#1331)

- Restore correct list of styles to markup modal
- DRY lists of styles

Fix #1330

lib/supported-features.js

@@ -0,0 +1,20 @@
+'use strict';
+
+const advertisedStyles = [
+  'plastic',
+  'flat',
+  'flat-square',
+  'for-the-badge',
+  'social',
+];
+
+const validStyles = advertisedStyles.concat(['default', '_shields_test']);
+
+function isValidStyle(style) {
+  return style ? validStyles.indexOf(style) >= 0 : false;
+}
+
+module.exports = {
+  advertisedStyles,
+  isValidStyle,
+};