Nice to have feature - Toggle the display of a marker's popup on click

## Hi guys,

Thanks for the job your doing! I was searching for an open source framework to build interactive maps and here it is Leaflet! :+1:!

I'm pretty new with it but I came through a simple - yet disturbing - behavior.

Indeed when interacting with interactive maps in my browser I most commonly close marker's popups by simply clicking on the marker itself. I thought that it would be a nice to have minor feature so I implemented and specified it in this 1af9769  commit.

Hope I can add some contribution to this cool project =)

Have a nice week!
Paul