Add polyfill for Promise ( only )

This is needed for whatwg-fetch + IE11.
An alternative is to include "node_modules/whatwg-fetch" in the
transform-runtime.
But my guess is that someone is likely going to add a lib that in turn
uses Promises, without adding it to the whitelist. This is safter.

src/polyfills.js

+// Promise global, Used ( at least ) by 'whatwg-fetch'. And required by IE 11
+require("core-js/fn/promise")

webpack-dist-bundle.config.js

@@ -13,6 +13,7 @@ module.exports = require('./make-webpack-config.js')({
 
   entry: {
     'swagger-ui-bundle': [
+      './src/polyfills',
       './src/core/index.js'
     ]
   },

webpack-dist-standalone.config.js

@@ -13,6 +13,7 @@ module.exports = require('./make-webpack-config.js')({
 
   entry: {
     'swagger-ui-standalone-preset': [
+      './src/polyfills',
       './src/standalone/index.js'
     ]
   },

webpack-dist.config.js

@@ -15,6 +15,7 @@ module.exports = require("./make-webpack-config.js")({
   entry: {
     "swagger-ui": [
       "./src/style/main.scss",
+      "./src/polyfills",
       "./src/core/index.js"
     ]
   },

webpack-hot-dev-server.config.js

@@ -10,10 +10,12 @@ module.exports = require("./make-webpack-config")({
 	devtool: "eval",
   entry: {
     'swagger-ui-bundle': [
+      './src/polyfills',
       './src/core/index.js'
     ],
     'swagger-ui-standalone-preset': [
       'webpack/hot/dev-server',
+      './src/polyfills',
       './src/standalone/index.js',
     ]
   },