Fix path issue on windows, when mix "/" and "\" with css-loader

[systembugtj]

__dirname + "/app" won't work well on windows when deal with css-loader, may be css-loader issue. use path.join to let the path friendly on window. It should also work on Mac or linux.

Some related error from windows

ERROR in ./app/styles.css
Module parse failed: D:\tests\react-web-app\app\styles.css Line 1: Unexpected token :
You may need an appropriate loader to handle this file type.
| :global *,
| :global *:after,
| :global *:before {
@ ./app/index.js 11:17-40

ERROR in ./~/normalize.css/normalize.css
Module parse failed: D:\tests\react-web-app\node_modules\normalize.css\normalize.css Line 9: Unexpected token {
You may need an appropriate loader to handle this file type.
| /
|
| html {
| font-family: sans-serif; / 1 /
| -ms-text-size-adjust: 100%; / 2 */
@ ./app/index.js 9:0-24

ERROR in ./app/components/Hello/Hello.css
Module parse failed: D:\tests\react-web-app\app\components\Hello\Hello.css Line 1: Unexpected token .
You may need an appropriate loader to handle this file type.
| .greeting {
| color: #00F;
| }
@ ./app/components/Hello/Hello.js 29:16-38