Support adding popups in map features

I've encountered a use-case where I need to add points and stand-alone popups to a Leaflet map. This is possible with the Leaflet API (documentation) and I believe this would be a useful feature in this module. Below is a patch which adds this to the current 7.x-1.x branch.

This works by specifying a feature type of "popup" and "content" for the content of the popup. I decided on the "content" property solely because the code detects if the feature has a popup and tries to bindPopup, which doesn't exist for L.Popup(), but this could definitely be changed if there is a better way to accomplish it in the future.