City codes?

This is a great package! Quick question about how should we get codes for each city? For example, I want to get google trends data of a keyword for all US cities supported by the package. Then I can loop through all the cities by obtaining their trends. 

I only saw `gtrends(keyword = "obama", geo = "US-AL-630")` given as an example and wonder if we have a list of all cities. Thanks!