Close #542 - Add an option for disabling running locally installed executables by default

test/test_ale_info.vader

@@ -88,6 +88,7 @@ Before:
   \  'let g:ale_sign_warning = ''--''',
   \  'let g:ale_statusline_format = [''%d error(s)'', ''%d warning(s)'', ''OK'']',
   \  'let g:ale_type_map = {}',
+  \  'let g:ale_use_global_executables = v:null',
   \  'let g:ale_warn_about_trailing_blank_lines = 1',
   \  'let g:ale_warn_about_trailing_whitespace = 1',
   \]