editorconfig: set max_line_length

See [1]. CONTRIBUTING.md says:

> Try to keep your lines under 80 columns, but you can go up to 100 if it
> improves readability.

[1]: https://github.com/editorconfig/editorconfig/wiki/EditorConfig-Properties#max_line_length

.editorconfig

@@ -7,6 +7,7 @@ charset = utf-8
 trim_trailing_whitespace = true
 indent_style = tab
 indent_size = 4
+max_line_length = 80
 
 [*.xml]
 indent_style = space