Allow usage of & at the end of a selector

[jleclanche]

Example:

.container {
    .element {
        background-color: blue;
        .active& {
            background-color: red;
        }
    }
}

Would result in:

.container .element {
    background-color: blue;
}

.active.container .element {
    background-color: red;
}

Right now it's a syntax error.

[jleclanche]

This is kind of an addendum to issue #286

[nex3]

This will be covered by the changes in #286.