Add an editorconfig file

From http://editorconfig.org/:
"EditorConfig helps developers define and maintain consistent coding
styles between different editors and IDEs. The EditorConfig project
consists of a file format for defining coding styles and a collection of
text editor plugins that enable editors to read the file format and
adhere to defined styles."

I tried my best to derive the specific settings from the Leaflet source
code.
I'm a bit unsure whether Leaflet uses lf or crlf though, there seems to
be a bit of both. :)