add an option to retain old names if mapping is missing

Author: Supriya S

src/getClassName.js

@@ -98,10 +98,13 @@ export default (styleNameValue: string, styleModuleImportMap: StyleModuleImportM
         if (handleMissingStyleName === 'throw') {
           throw new Error('Could not resolve the styleName \'' + styleName + '\'.');
         }
-        if (handleMissingStyleName === 'warn') {
+        else if (handleMissingStyleName === 'warn') {
           // eslint-disable-next-line no-console
           console.warn('Could not resolve the styleName \'' + styleName + '\'.');
         }
+        else if (handleMissingStyleName === 'retain') {
+          return styleName;
+        }
       }
 
       return styleModuleMap[styleName];

src/schemas/optionsSchema.json

@@ -52,7 +52,7 @@
       "type": "boolean"
     },
     "handleMissingStyleName": {
-      "enum": ["throw", "warn", "ignore"]
+      "enum": ["throw", "warn", "ignore", "retain"]
     },
     "attributeNames": {
       "additionalProperties": false,