Add bounds option to configuration. Fixes #482.

This commit allows people to specify a default bounds in the configuration when using Google's service. Any lookup will include the bounds, which adds support for bounds in the `near` method (see [issue #482](https://github.com/alexreisner/geocoder/issues/482)). If bounds are specified in the config as well as in a search query (like `Geocoder.search("Paris", :bounds => [[32.1,-95.9], [33.9,-94.3]])`), then the bounds specified in the search will take precedence over those in the configuration.