Add option to output .js files while preserving jsx

This commit adds the ability to preserve jsx in source code, but
also to output .js files rather than .jsx files. This is useful
for react-native which does not support .jsx files.

src/compiler/commandLineParser.ts

@@ -67,10 +67,11 @@ namespace ts {
             name: "jsx",
             type: createMapFromTemplate({
                 "preserve": JsxEmit.Preserve,
+                "preservewithjsextension": JsxEmit.PreserveWithJsExtension,
                 "react": JsxEmit.React
             }),
             paramType: Diagnostics.KIND,
-            description: Diagnostics.Specify_JSX_code_generation_Colon_preserve_or_react,
+            description: Diagnostics.Specify_JSX_code_generation_Colon_preserve_preserveWithJsExtension_or_react,
         },
         {
             name: "reactNamespace",