Add more map providers

[JoeGermuska]

While StoryMapJS doesn't directly use Leaflet-providers, several of the basemap layers documented on http://leaflet-extras.github.io/leaflet-providers/preview/index.html would work with StoryMapJS.

Many of them require an API key, which is probably not worth trying to integrate, but for those that don't need an extra key, we should be able to add the URLs to the options and give authors a few more choices.