Update get_googlemap.R to include api_key support

I'm behind a firewall at work, and we exhaust our usage limits Google Maps API Web Services regularly. The changes I've proposed here allow an api_key to be specified within the get_googlemap function call. This then gets passed to an api_url --> post_url. If no api_key is specified, it is treated the same as if other specifications (e.g., style, region, language) are missing.