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

test/test_history_saving.vader

@@ -16,6 +16,7 @@ Before:
   \})
 
 After:
+  let g:ale_history_enabled = 1
   unlet g:history
   let g:ale_buffer_info = {}
   let g:ale_max_buffer_history_size = 20
@@ -40,6 +41,16 @@ Execute(History should be set when commands are run):
   " The Job ID will change each time, but we can check the type.
   AssertEqual type(1), type(g:history[0].job_id)
 
+Execute(History should be not set when disabled):
+  AssertEqual 'foobar', &filetype
+
+  let g:ale_history_enabled = 0
+
+  call ale#Lint()
+  call ale#engine#WaitForJobs(2000)
+
+  AssertEqual 0, len(g:ale_buffer_info[bufnr('%')].history)
+
 Execute(History items should be popped after going over the max):
   let g:ale_buffer_info[1] = {
   \ 'history': map(range(20), '{''status'': ''started'', ''job_id'': v:val, ''command'': ''foobar''}'),