Allow consumers to override Rollup warning handler

Let the `onwarn` handler set by downstream projects in their Rollup configs
override the default one used by the `buildJS` and `watchJS` functions exported
by this project.

lib/rollup.js

@@ -27,8 +27,8 @@ export async function buildJS(rollupConfig) {
   await Promise.all(
     configs.map(async config => {
       const bundle = await rollup.rollup({
-        ...config,
         onwarn: logRollupWarning,
+        ...config,
       });
       await bundle.write(config.output);
     }),
@@ -47,8 +47,8 @@ export async function watchJS(rollupConfig) {
 
   const watcher = rollup.watch(
     configs.map(config => ({
-      ...config,
       onwarn: logRollupWarning,
+      ...config,
     })),
   );