Add readonly to FormControl formstate

**I'm submitting a ...** (check one with "x")

```
[ ] bug report => search github for a similar issue or PR before submitting
[x] feature request
[ ] support request => Please do not submit support request here, instead see https://github.com/angular/angular/blob/master/CONTRIBUTING.md#question
```

Currently there is a "disabled" flag which will disable the control, and also disable the validator.
But there also should be a "readonly" flag which do not disable the validator.

**Current behavior**
- Does not exist -

**Expected behavior**
- Explained above -

**What is the motivation / use case for changing the behavior?**

**Please tell us about your environment:**
- **Angular version:** 2.0.0-rc.6
- **Browser:** [all]
- **Language:** [all]
- **Node (for AoT issues):** `node --version` = 6.5.0

該 issue 未指定檔案、測試或進入點。先找出 Angular 的 FormControl 表單狀態實作，以及現有的 disabled 狀態涵蓋範圍；當支援 readonly 狀態、同時維持驗證有效，且不變更 disabled 行為時，即表示完成。

由索引模型根據 Issue 內容生成。