Added an .editorconfig file

This allows most IDEs and editors (with the proper plugin) to
automatically use the correct identation and line endings.

In short, you'll never ever have to worry about tabs or line breaks in your
code again. In additon, this won't have any effect on your other files or
projects, who'll continue to use your own global or custom settings.

There are plugins available for Microsoft Visual Studio, Notepad++, gedit, vim,
and many other popular editors and IDEs.

More information can be found on http://www.editorconfig.org/.