add an .editorconfig (#233)

Author: emberian

.editorconfig

@@ -0,0 +1,11 @@
+root = true
+
+[*]
+end_of_line = lf
+indent_style = space
+charset = utf-8
+
+[*.{ml,mli}]
+insert_final_newline = true
+trim_trailing_whitespace = true
+indent_size = 2