Create .editorconfig

editorconfig is always useful to have to help ensure consistent code formatting.

This config provides good defaults and is copied from:
https://github.com/twbs/bootstrap/blob/master/.editorconfig

.editorconfig

@@ -0,0 +1,11 @@
+# editorconfig.org
+
+root = true
+
+[*]
+charset = utf-8
+end_of_line = lf
+indent_size = 2
+indent_style = space
+insert_final_newline = true
+trim_trailing_whitespace = true