Configurable Google Maps options with defaults

As a user I should be able to configure map [options](https://developers.google.com/maps/documentation/javascript/3.exp/reference#MapOptions)

Options can be set globally via `_config.yml`:

``` yaml
maps:
  google:
    options:
      disableDefaultUI: false
      scrollwheel: false
```

or it can be set from tag:

```
{% google_map on_page zoom:10 %}
```
