Added beforeSend option to customize ajax requests

I added `beforeSend` option to allow ajax request modifications. The main motivation for this was the ability to add HTTP headers expected by backend.

The parameters correspond with beforeSend method from jQuery:

    $('#foo').magicSuggest({
        ...
        beforeSend: function(xhr) {
            xhr.setRequestHeader('Authorization', 'API token');
        },
        ...
    });