569 support vim sign group and priority (#2786)

* Use sign-group only on supported vim versions.

The sign-group feature is only available in nvim 0.4.0 and vim 8.1.614.

* Add priority to ALE signs.

This allows users to set a priority to ALE signs to take precedence over
other plugin signs.

test/test_setting_problems_found_in_previous_buffers.vader

@@ -38,7 +38,7 @@ After:
   " Items and markers, etc.
   call setloclist(0, [])
   call clearmatches()
-  sign unplace *
+  call ale#sign#Clear()
 
 Given foobar(A file with some lines):
   foo