Add support for beforeSend callback

[zokioki]

This allows ahoy to be configured with an optional beforeSend callback, allowing the user to modify a request before sending if needed. For example:

ahoy.configure({
  beforeSend: (xhr, settings) => {
    // do something with request ...
  }
});

One use case for this is to allow for the setting of custom headers on a per-request basis.