add sourceType config option

Valid values are `module` (default) and `script`. Using `script` will suppress implied strict mode and will prevent you from using ES2015 modules.

lib/jsdoc/config.js

@@ -30,6 +30,7 @@ var defaults = {
         includePattern: '.+\\.js(doc|x)?$',
         excludePattern: ''
     },
+    sourceType: 'module',
     tags: {
         allowUnknownTags: true,
         dictionaries: ['jsdoc', 'closure']

lib/jsdoc/src/astbuilder.js

@@ -1,12 +1,13 @@
 'use strict';
 
 var babylon = require('babylon');
+var env = require('jsdoc/env');
 var logger = require('jsdoc/util/logger');
 
 // exported so we can use them in tests
 var parserOptions = exports.parserOptions = {
     ranges: true,
-    sourceType: 'module',
+    sourceType: env.conf.sourceType,
     plugins: [
         'asyncGenerators',
         'bigInt',