function getExtensionURL(id, version) {
        return StringUtils.format(brackets.config.extension_url, id, version);
    }
Given an extension id and version number, returns the URL for downloading that extension from
the repository. Does not guarantee that the extension exists at that URL.
@param {string} id The extension's name from the metadata.
@param {string} version The version to download.
@return {string} The URL to download the extension from.