loop helper

now loop variable available in for,each loops

![2013-05-11 18 37 40](https://f.cloud.github.com/assets/1452141/491633/5fbe3c96-ba48-11e2-83c5-3f93d73a6274.png)

all is simple, I used literal @ for exposing loop length to loop helper, because it's very hard parse with regexp without marking it

some examples:

jade: 
```jade
- length = 3
ul
  for (var i = 0; i < @length; i++)
    li(class=loop.first && 'active')
```

html: 
```html
<ul>
  <li class="active"></li>
  <li></li>
  <li></li>
</ul>
```

jade:
```jade
each user in ["Petya", "Masha", "Sveta"]
  div #{user}
    if (loop.first)
      | - is man
    if (loop.index == 2)
      | - is girl
    if (loop.last)
      | - is girl
```

html: 
```html
<div>Petya- is man
</div>
<div>Masha- is girl
</div>
<div>Sveta- is girl
```

jade: 
```jade
for (var i = 0; i < @5; i++)
  div loop.index0, element № 3 expected, element № #{i}
    - if (loop.index0 == 3)
      |  marked
```

html: 
```html
<div>element № 3 expected as marked, element № 0
</div>
<div>element № 3 expected as marked, element № 1
</div>
<div>element № 3 expected as marked, element № 2
</div>
<div>element № 3 expected as marked, element № 3 marked
</div>
<div>element № 3 expected as marked, element № 4
</div>
```