Add license headers in js/dist files

By implementing the same approach of rollup.config.js
and replicate it in build-plugins.js, individual plugins
will display license headers.

build/rollup.config.js

@@ -1,10 +1,9 @@
 const path    = require('path')
 const babel   = require('rollup-plugin-babel')
 const resolve = require('rollup-plugin-node-resolve')
+const banner  = require(path.resolve(__dirname, './banner.js'))
 
-const pkg     = require(path.resolve(__dirname, '../package.json'))
 const BUNDLE  = process.env.BUNDLE === 'true'
-const year    = new Date().getFullYear()
 
 let fileDest  = 'bootstrap.js'
 const external = ['jquery', 'popper.js']
@@ -36,11 +35,7 @@ if (BUNDLE) {
 module.exports = {
   input: path.resolve(__dirname, '../js/src/index.js'),
   output: {
-    banner: `/*!
-  * Bootstrap v${pkg.version} (${pkg.homepage})
-  * Copyright 2011-${year} ${pkg.author}
-  * Licensed under MIT (https://github.com/twbs/bootstrap/blob/master/LICENSE)
-  */`,
+    banner,
     file: path.resolve(__dirname, `../dist/js/${fileDest}`),
     format: 'umd',
     globals,