Support for xhr

[frankbaele]

Are you willing to add xhr support for getting the midi files? The file upload api is a little limiting. In my current application im solving this by the following snippet:

    function getMidiStream(url, callback) {
        var xhr = new XMLHttpRequest();
        xhr.open('GET', url);
        xhr.overrideMimeType('text/plain; charset=x-user-defined');
        xhr.onreadystatechange = function () {
            if (this.readyState === 4) {
                if (this.status === 200) {
                    var t = this.responseText || '';
                    var ff = [];
                    var mx = t.length;
                    var scc = String.fromCharCode;
                    for (var z = 0; z < mx; z++) {
                        ff[z] = scc(t.charCodeAt(z) & 255);
                    }
                    callback(ff.join(''));
                } else {
                    onerror && onerror('Unable to load MIDI file');
                }
            }
        };
        xhr.send();
    }

    getMidiStream(url, function (stream) {
           var midi = MidiConvert.parseParts(stream, options);
    })

I wanted to create a pull request but had no idea where to put the code. Btw this code is heavily inspired by how midijs solves this. Its kind of a hack to get a blob.

Frank