String Literal autocomplete support

Thanks for https://github.com/Microsoft/TypeScript/pull/5185 :rose:. Would be great if we could get autocomplete support for string literals 

``` ts
type Foo = "aReallyLong" | "bReallyLong";
let foo:Foo;
foo = "aReallyLong"; // allow me to autocomplete this
```

refs https://github.com/Microsoft/TypeScript/issues/5554#issuecomment-155622379
