Resolve imported CSS files from node_modules (#105)

* Resolve imported CSS files from anywhere
* Resolve imported CSS files from src and node_modules folder only

Closes #103

config/webpack.config.dev.js

@@ -67,7 +67,7 @@ module.exports = {
       },
       {
         test: /\.css$/,
-        include: srcPath,
+        include: [srcPath, nodeModulesPath],
         loader: 'style!css!postcss'
       },
       {

config/webpack.config.prod.js

@@ -63,7 +63,7 @@ module.exports = {
       },
       {
         test: /\.css$/,
-        include: srcPath,
+        include: [srcPath, nodeModulesPath],
         // Disable autoprefixer in css-loader itself:
         // https://github.com/webpack/css-loader/issues/281
         // We already have it thanks to postcss.