Ask whether users want to load back last session or not

Hello :)

At the moment, this great tool automatically loads last session's data when accessing http://jsoneditoronline.org/

That is useful most of the time, however when last session was painfully heavy to parse (e.g. too much data) accessing the site is quite long because it loads the painful data again, even if we probably don't want it back.

My suggestion would be to ask the user whether last session should be loaded or not (for instance http://geojson.io/ does that), or to provide a button in the top bar offering a way to load latest session if the user wishes to restore it.

That'd be a cool user experience improvement I think :)

_Great tool otherwise, it really is useful :+1:_