Add method to only make links

```
It would be great if the Markdown.Converter had a method which only created
links. I'm dealing with feed posts and I don't want the user to be able to
create headers and all that jazz.

```

Original issue reported on code.google.com by `mks.coet...@gmail.com` on 2 Feb 2012 at 9:19

查看程式碼庫中的 Markdown.Converter 類別，以了解它目前如何處理 markdown。找出連結建立邏輯與其他格式設定邏輯分開的位置。建立一個新方法，僅隔離並套用連結轉換規則。透過驗證標題和其他 markdown 會被忽略，而連結會正確產生來進行測試。

由索引模型根據 Issue 內容生成。