Cause SimpleXml to Escape Text Nodes as Needed

In its current form, if `$config[$x]` contains ampersands, the script will fail. This PR fixes the issue using the fact that `SimpleXMLElement::value` assignments automatically get escaped (whereas `SXE::addChild($x, $y)` does not).

```
in /Users/smithm5/Documents/dev/MagentoTarToConnect/magento-tar-to-connect.php on line 365
PHP Stack trace:
PHP   1. {main}() /Users/smithm5/Documents/dev/MagentoTarToConnect/magento-tar-to-connect.php:0
PHP   2. main() /Users/smithm5/Documents/dev/MagentoTarToConnect/magento-tar-to-connect.php:397
PHP   3. create_extension_xml() /Users/smithm5/Documents/dev/MagentoTarToConnect/magento-tar-to-connect.php:291
PHP   4. build_extension_xml() /Users/smithm5/Documents/dev/MagentoTarToConnect/magento-tar-to-connect.php:347
PHP   5. SimpleXMLElement->addChild() /Users/smithm5/Documents/dev/MagentoTarToConnect/magento-tar-to-connect.php:365

Warning: SimpleXMLElement::addChild(): unterminated entity reference
```