Add editor config

Configure indent style and spaces across different editors. This should
make contributions a tiny bit easier.

.editorconfig

@@ -0,0 +1,11 @@
+# EditorConfig is awesome: http://EditorConfig.org
+
+# top-most EditorConfig file
+root = true
+
+# Unix-style newlines with a newline ending for every file
+# Indent with 4 spaces
+[php]
+end_of_line = lf
+indent_style = space
+indent_size = 4