Support for OR based queries

HI,
    First of all, thanks  @olivernn for your great work. Currently lunr `search` do `AND` queries for multiple terms. But the app i am working on requires OR queries. I am trying to integrate OR based queries to lunr. Its fairly straight forward returning the result set. By replacing the

    var documentSet = documentSets.reduce(function (memo, set) {   
        return memo.intersect(set)
    })

with

    var documentSet = documentSets.reduce(function (memo, set) {   
        return memo.union(set)
    })

in `lunr.Index.prototype.search` would do the trick. I have ran some tests and it seems ok. I have added few test cases too. 

But my main concern is about the score calculation for OR queries because  i am not very sure about how to test this specific case. If you think this is worth, have a look at it.