[WIP] Generate an hash for cache bypassing between builds (#291)

As things can get pretty complex while explaining, gonna apply here
things proposed over this branch.

lib/utils/webpack.config.js

@@ -8,6 +8,9 @@ import _ from 'lodash'
 
 import babelConfig from './babel-config'
 
+// Generate an hash for cache bypassing between builds (#291)
+const ASSETS_CACHE = (+new Date()).toString(32)
+
 let modifyWebpackConfig
 try {
   const gatsbyNodeConfig = path.resolve(process.cwd(), './gatsby-node')
@@ -39,21 +42,21 @@ module.exports = (program, directory, stage, webpackPort = 1500, routes = []) =>
         // Webpack will always generate a resultant javascript file.
         // But we don't want it for this step. Deleted by build-css.js.
         return {
-          path: `${directory}/public`,
           filename: 'bundle-for-css.js',
+          path: `${directory}/public/${ASSETS_CACHE}`,
         }
       case 'build-html':
         // A temp file required by static-site-generator-plugin. See plugins() below.
         // Deleted by build-html.js, since it's not needed for production.
         return {
-          path: `${directory}/public`,
           filename: 'render-page.js',
+          path: `${directory}/public`,
           libraryTarget: 'umd',
         }
       case 'build-javascript':
         return {
           filename: 'bundle.js',
-          path: `${directory}/public`,
+          path: `${directory}/public/${ASSETS_CACHE}`,
         }
       default:
         throw new Error(`The state requested ${stage} doesn't exist.`)
@@ -103,6 +106,7 @@ module.exports = (program, directory, stage, webpackPort = 1500, routes = []) =>
           new webpack.DefinePlugin({
             'process.env': {
               NODE_ENV: JSON.stringify(process.env.NODE_ENV ? process.env.NODE_ENV : 'production'),
+              ASSETS_CACHE: JSON.stringify( ASSETS_CACHE ),
             },
             __PREFIX_LINKS__: program.prefixLinks,
           }),
@@ -114,7 +118,8 @@ module.exports = (program, directory, stage, webpackPort = 1500, routes = []) =>
           new webpack.DefinePlugin({
             'process.env': {
               NODE_ENV: JSON.stringify(process.env.NODE_ENV ? process.env.NODE_ENV : 'production'),
-            },
+              ASSETS_CACHE: JSON.stringify( ASSETS_CACHE ),
+          },
             __PREFIX_LINKS__: program.prefixLinks,
           }),
         ]
@@ -127,6 +132,7 @@ module.exports = (program, directory, stage, webpackPort = 1500, routes = []) =>
           new webpack.DefinePlugin({
             'process.env': {
               NODE_ENV: JSON.stringify(process.env.NODE_ENV ? process.env.NODE_ENV : 'production'),
+              ASSETS_CACHE: JSON.stringify( ASSETS_CACHE ),
             },
             __PREFIX_LINKS__: program.prefixLinks,
           }),