Add support for setting base directories of assets

Created by: ghost

Sometimes, we need to deploy our code on lousy service provider. The service provider supports flat directory only, rather than hierarchical directory.

In above case, building code with create-react-app predefined hierarchical directory make us agonizing.

Service prodivder supports:

├── asset-manifest.json
├── favicon.ico
├── index.html
├── manifest.json
├── service-worker.js
├── main.f58a13b4.css
├── main.f58a13b4.css.map
├── main.1a92a0e5.js
└── main.1a92a0e5.js.map

But creact-react-app outputs:

├── asset-manifest.json
├── favicon.ico
├── index.html
├── manifest.json
├── service-worker.js
└── static
    ├── css
    │   ├── main.f58a13b4.css
    │   └── main.f58a13b4.css.map
    ├── js
        ├── main.1a92a0e5.js
        └── main.1a92a0e5.js.map

So I opened a PR to make setting base directories of assets possible.

This PR does simple things:

• export 3 configurable environment variables: CSS_DIR / JS_DIR / MEDIA_DIR
• configure output path with above 3 environment variables.

If I should do some other things to make it merged, please comment. 😺