Add .editorconfig (#1451)

.editorconfig

@@ -0,0 +1,16 @@
+# editorconfig.org
+
+root = true
+
+[*]
+charset = utf-8
+indent_size = 4
+indent_style = space
+insert_final_newline = true
+trim_trailing_whitespace = true
+
+[*.js]
+indent_size = 2
+
+[*.md]
+trim_trailing_whitespace = false