Add an option for completely disabling command history, and add documentation

plugin/ale.vim

@@ -134,6 +134,9 @@ let g:ale_warn_about_trailing_whitespace =
 " A flag for controlling the maximum size of the command history to store.
 let g:ale_max_buffer_history_size = get(g:, 'ale_max_buffer_history_size', 20)
 
+" A flag for enabling or disabling the command history.
+let g:ale_history_enabled = get(g:, 'ale_history_enabled', 1)
+
 function! s:ALEInitAuGroups() abort
     augroup ALERunOnTextChangedGroup
         autocmd!