[Request] Extend tsconfig via a package

**TypeScript Version:**  2.3.2

**Code**

```ts
{
  "extends": "tsconfig-react-app"
}
```

**Expected behavior:**

Using the node module resolution, finds the specified package and inspects the `package.json`'s `main` field for a `.json` file, which is used to extend the configuration.

**Actual behavior:**
Something along the lines "must be a relative path".
