feat: add core ES module. (#6287)

Allows you to do `import videojs from 'video.js/core.es.js';` to go along with `require('video.js/core.js')`;

.gitignore

@@ -34,6 +34,7 @@ es5/*
 .idea/
 
 core.js
+core.es.js
 
 # Ignore Chinese clones for now.
 lang/zh-Han*.json

.npmignore

@@ -5,3 +5,4 @@ dist/video-js-*.zip
 !es5/**
 !src/css/**
 !core.js
+!core.es.js

package.json

@@ -158,7 +158,8 @@
       "sandbox",
       "test/dist",
       "test/api",
-      "core.js"
+      "core.js",
+      "core.es.js"
     ]
   },
   "greenkeeper": {

rollup.config.js

@@ -186,16 +186,24 @@ export default cliargs => [
     onwarn,
     watch
   },
-  // core
+  // core cjs, es
   {
     input: 'src/js/video.js',
-    output: {
-      format: 'cjs',
-      file: 'core.js',
-      strict: false,
-      banner,
-      globals: globals.module
-    },
+    output: [
+      {
+        format: 'es',
+        file: 'core.es.js',
+        strict: false,
+        banner,
+        globals: globals.module
+      }, {
+        format: 'cjs',
+        file: 'core.js',
+        strict: false,
+        banner,
+        globals: globals.module
+      }
+    ],
     external: externals.module,
     plugins: [
       json(),