Add .editorconfig to maintain consistent coding style (#3)

This config file of format rules can be applied by many IDEs and editors,
e.g. codium and vim. Some need an extra plugin for that, like the latter.

.editorconfig

@@ -0,0 +1,14 @@
+root=true
+
+[*]
+charset=utf-8
+end_of_line=lf
+insert_final_newline=true
+trim_trailing_whitespace=true
+indent_style=space
+indent_size=4
+continuation_indent_size=8
+
+[{*.yml,*.yaml}]
+indent_style=space
+indent_size=2