Supported newlines

I tried to compile the following haml:

```haml
%head
  %title

    hi
```

expected:

```html
<head>
<title>hi</title></head>
```

but actual:

```sh
HamlError: (Haml):4 invalid indentation; got 2, when previous was 0
```

This patch supports newlines and lines filled in whitespace only.