Allow specifying custom Curl options in curl_options config parameter

[hackel]

Also added error handling for Curl failures. Still need to add this to documentation if approved. I did this because I wanted to use a proxy server when adding an attachment by URL. It is very straightforward: Example config file (stapler.php):

    'curl_options' => [
        CURLOPT_PROXY => '127.0.0.1:3128',
        CURLOPT_SSL_VERIFYHOST => false,
    ],