Support for '&'

Currently jLess doesn't parse '&' sign, which means last selector extension (by class or property) without level bump, e.g.:

```
a {
    text-decoration: none;
    &:hover, &.selected li, .btn{
        border-width: 1px;
    }
}
```

expands to:

```
a{
    text-decoration:none;
}
a:hover, a.selected li, a .btn{
    border-width:1px;
}
```

It would be nice if jless will support this feature.
