First Pull Request - Added a close button.

I've never done a pull request so I'm not sure how this is done. But I made some additions to your leaflet sidebar v2 project. By looking at your previous version I was able to pull out the necessary functions to implement it in sidebar v2. So the leaflet-sidebar.js file and the _base.scss file are changed so that you have a button that will collapse the sidebar. 

It was able to run through gulp and seemed fine. I would have included all the files but I was having some issues with LF and CRLF files and I thought the source files would be enough. Let me know if this works. I haven't done too much testing but it seems to work for me.