Option to open lists vertically (#1381)

* Add configuration option to open lists vertically

* Add tests, clean up vertical list config

* Vertical list option cleanup

* Use is# for tests
* Order properties in documentation alphabetically

autoload/ale/debugging.vim

@@ -29,6 +29,7 @@ let s:global_variable_list = [
 \    'ale_linters',
 \    'ale_linters_explicit',
 \    'ale_list_window_size',
+\    'ale_list_vertical',
 \    'ale_loclist_msg_format',
 \    'ale_max_buffer_history_size',
 \    'ale_max_signs',