Expose path to Draft.css for usage by CSS pre-processors

Summary:
**what is the change?:**
CREDIT TO darobin for this change, originally made in
#566

 ---

Some CSS pre-processors understand node_modules and are able to import without requiring an absolute path. However, without assistance from the module all that they can resolve is its root directory. This leads to import statements like the following:

```
import "draft-js/dist/Draft.css";
```

While this works, it requires internal knowledge of the package's layout. This requires digging around and if the layout changes, this breaks.

These pre-processors also recognise a style field in package.json, which is the equivalent of main for CSS. This allows one to import thusly:

```
import "draft-js";
```

It's a small change, but it helps!
Closes #1442

Differential Revision: D6088534

fbshipit-source-id: fcede534a18620907adfb6a30391b9f722756e8c

package.json

@@ -17,6 +17,7 @@
     "PATENTS"
   ],
   "main": "lib/Draft.js",
+  "style": "dist/Draft.css",
   "repository": "facebook/draft-js",
   "license": "BSD-3-Clause",
   "scripts": {