support for extra initialization options

Hi,

I needed something a bit custom and I noticed I had now way of altering all of the options you were passing to the map initializer, so I added an extra option for your plugin that extends the map options with whatever I pass along.

This should be documented on your site and specially note that if you add options that you are currently setting they will be overwritten.

This is what I was trying to accomplish:

````javascript
$('#map_canvas').gMap({
	address: settings.map_address,
	zoom: 14,
	markers: [
		{ 'address' : settings.map_address }
	],
	extra: {
		mapTypeControlOptions = {
			mapTypeIds: ['map_style', google.maps.MapTypeId.ROADMAP, google.maps.MapTypeId.SATELLITE, google.maps.MapTypeId.HYBRID],
		}		
	}
});
````

which now I can do it. I haven't touched the minified version so let you do that if you want to merge this in.