Multiline strings

Got idea from #9 

So, multiline strings are currently supported.

``` cfg
multi = "some
string
with
newlines";
```

Might be nice to maybe stripe whitespace from the left so that:

``` cfg
section {
  multi = "some
  multi
  line
  string";
}
```

Will be:

```
some
multi
line
string
```

Instead of:

```
some
  multi
  line
  string
```
