support some methods to return all docs when text is empty

[bloodynumen]

Like this:

searcher.Search(types.SearchReq{
    TextMayBeEmpty: true,
    Text: "",
    RankOpts: &types.RankOpts{
        OutputOffset: 0,
        MaxOutputs:   100,
    },
})

It's useful for providing web api

[vcaesar]

You can use spaces " " to return all docs:

searcher.Search(types.SearchReq{
    Text: " ",
    RankOpts: &types.RankOpts{
        OutputOffset: 0,
        MaxOutputs:   100,
    },
})

[bloodynumen]

@vcaesar thank you~