Fix #171 - Implement basic error highlighting

autoload/ale/cleanup.vim

@@ -10,6 +10,11 @@ function! ale#cleanup#Buffer(buffer) abort
             call ale#engine#ClearJob(l:job)
         endfor
 
+        " Clear delayed highlights for a buffer being removed.
+        if g:ale_set_highlights
+            call ale#highlight#UnqueueHighlights(a:buffer)
+        endif
+
         call remove(g:ale_buffer_info, a:buffer)
     endif
 endfunction