Add editorconfig

Provide a basic editorconfig file with this repository's defaults.

.editorconfig

+root = true
+
+[*]
+charset = utf-8
+indent_size = 4
+indent_style = space
+insert_final_newline = true
+trim_trailing_whitespace = true
+
+[Makefile]
+indent_style = tab
+
+[*.{yml,yaml}]
+indent_size = 2