Add a way for translators to pass arguments to callbacks for HTTP requests

We often want to pass arguments from "multiples" pages to the pages for individual items. Currently this is typically done (sometimes incorrectly) by looping through items and doing processDocuments/doGet calls for each url. This produces a (sometimes) undesired effect of performing parallel HTTP requests instead of the typical sequential requests that happens when passing an array of URLs to these functions.

This patch allows arguments to be passed to the callback function and also keeps the sequential HTTP requests.

Also, looks like `responseCharset` was getting lost in doGet calls with multiple URLs