Adding promise support

[samuelfaj]

I've just added promise support.
Now you have a final callback.

const scraper = require("samuelfaj-google-scraper");
var options = {
    query: 'nodejs',
    limit: 10
};

scraper.search(options, function(err, url, meta) {
    // This is called for each result
    if(err) throw err;
    console.log(url);
    console.log(meta.title);
    console.log(meta.meta);
    console.log(meta.desc)
}).then((results) => {
    console.log("All searches ended", results);
});