[keysnail] use goo.gl to shorten current URL

keysnail-goo.gl.js

ext.add("goo.gl", function () {
    let endpoint = "https://www.googleapis.com/urlshortener/v1/url";
    let params = { "longUrl": window._content.document.location.href };
    let result = util.httpPostJSON(endpoint, params, function (xhr) {
        if (xhr.status == 200) {        
            var ret = JSON.parse(xhr.responseText);
            command.setClipboardText(ret.id);        
            display.echoStatusBar("Short URL copied into clipboard: " + ret.id, 3000);
        } else {
            display.echoStatusBar("goo.gl service failed: " + xhr.statusText, 3000);
        }
    });
}, "Shorten URL with http://goo.gl service");
 
// goo.gl only accepts content-type as 'application/json',
// but keysnail's httpGet/httpPost doesn't support it
util.httpPostJSON= function (url, params, callback) {
            let xhr = new XMLHttpRequest();
 
            switch (typeof params)
            {
            case "string":
                // nothing
                break;
            case "object":
                params = JSON.stringify(params)
                break;
            default:
                params = "";
                break;
            }
 
            let async = typeof callback === "function";
 
            if (async)
            {
                xhr.onreadystatechange = function () {
                    if (xhr.readyState === 4)
                        callback(xhr);
                };
            }
 
            xhr.open("POST", url, async);
 
            xhr.setRequestHeader("Content-type", "application/json");
            xhr.setRequestHeader("Content-length", params.length);
            xhr.setRequestHeader("Connection", "close");
 
            xhr.send(params);
 
            return xhr;
}