Add possibility to precompile as ES module

Author: jaylinski

bin/handlebars.js

@@ -11,22 +11,30 @@ const yargs = require('yargs')
     type: 'string',
     description: 'Source Map File'
   })
-  .option('a', {
-    type: 'boolean',
-    description: 'Exports amd style (require.js)',
-    alias: 'amd'
+  .option('esm', {
+    type: 'string',
+    description:
+      'Exports ECMAScript module style, path to Handlebars module (eg. "handlebars/lib/handlebars")',
+    default: null
   })
   .option('c', {
     type: 'string',
     description: 'Exports CommonJS style, path to Handlebars module',
     alias: 'commonjs',
     default: null
   })
+  .option('a', {
+    type: 'boolean',
+    description: 'Exports AMD style (require.js)',
+    alias: 'amd',
+    deprecated: true
+  })
   .option('h', {
     type: 'string',
-    description: 'Path to handlebar.js (only valid for amd-style)',
+    description: 'Path to handlebar.js (only valid for AMD-style)',
     alias: 'handlebarPath',
-    default: ''
+    default: '',
+    deprecated: true
   })
   .option('k', {
     type: 'string',

lib/precompiler.js

@@ -206,6 +206,8 @@ module.exports.cli = function(opts) {
       );
     } else if (opts.commonjs) {
       output.add('var Handlebars = require("' + opts.commonjs + '");');
+    } else if (opts.esm) {
+      output.add(`import handlebars from "${opts.esm}";`);
     } else {
       output.add('(function() {\n');
     }

spec/expected/help.menu.txt

@@ -5,9 +5,11 @@ Options:
   --help               Outputs this message                                                                    [boolean]
   -f, --output         Output File                                                                              [string]
   --map                Source Map File                                                                          [string]
-  -a, --amd            Exports amd style (require.js)                                                          [boolean]
+  --esm                Exports ECMAScript module style, path to Handlebars module (eg. "handlebars/lib/handlebars")
+                                                                                                [string] [default: null]
   -c, --commonjs       Exports CommonJS style, path to Handlebars module                        [string] [default: null]
-  -h, --handlebarPath  Path to handlebar.js (only valid for amd-style)                            [string] [default: ""]
+  -a, --amd            Exports AMD style (require.js)                                             [deprecated] [boolean]
+  -h, --handlebarPath  Path to handlebar.js (only valid for AMD-style)               [deprecated] [string] [default: ""]
   -k, --known          Known helpers                                                                            [string]
   -o, --knownOnly      Known helpers only                                                                      [boolean]
   -m, --min            Minimize output                                                                         [boolean]

spec/precompiler.js

@@ -197,6 +197,13 @@ describe('precompiler', function() {
     equal(/return Handlebars\.partials\[/.test(log), false);
     equal(/template\(amd\)/.test(log), true);
   });
+  it('should output es module templates', function() {
+    Handlebars.precompile = function() {
+      return 'esm';
+    };
+    Precompiler.cli({ templates: [emptyTemplate], esm: true });
+    equal(/template\(esm\)/.test(log), true);
+  });
   it('should output commonjs templates', function() {
     Handlebars.precompile = function() {
       return 'commonjs';