Add .editorconfig

This file tells the github online editor to use tabs of width 4 instead
of 8. Open
[this](https://github.com/JohannesLorenz/github_test/blob/master/test.cpp)
test file in your browser to see how it works (the width is 5 in the
example).

Actually, it also tells many other editors...

.editorconfig

@@ -0,0 +1,5 @@
+root = true
+
+[*]
+indent_style = tab
+tab_width = 4