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/banner.js

@@ -0,0 +1,11 @@
+const path  = require('path')
+const pkg   = require(path.resolve(__dirname, '../package.json'))
+const year  = new Date().getFullYear()
+
+module.exports = function () {
+  return `/*!
+  * Bootstrap v${pkg.version} (${pkg.homepage})
+  * Copyright 2011-${year} ${pkg.author}
+  * Licensed under MIT (https://github.com/twbs/bootstrap/blob/master/LICENSE)
+  */`
+}