Close #3285 - lint_file is now dynamic

`lint_file` can now be computed dynamically with a callback function,
which can return a deferred result, as per `ale#command#Run`. This
allows linters to dynamically switch between checking files on disk,
or checking code on the fly.

Some tests have been fixed on Windows.

test/smoke_test.vader

@@ -1,8 +1,10 @@
 Before:
+  Save g:ale_enabled
   Save g:ale_set_lists_synchronously
   Save g:ale_buffer_info
   Save &shell
 
+  let g:ale_enabled = 1
   let g:ale_buffer_info = {}
   let g:ale_set_lists_synchronously = 1