Support `//.` denoting a text only block comment

[closes #1134]

This adds the ability to have text only block comments using `.` just like after tags e.g.

```jade
//.
  User: Pu
  Date: 6/2/13
  Time: 4:13 PM
```

would compile to

```html
<!--User: Pu
Date: 6/2/13
Time: 4:13 PM-->
```

I'd like to hear people's thoughts before I propose merging this.  Is there a better way of implementing this?  Are there any suggestions for better syntax?