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.

doc/ale.txt

@@ -1543,6 +1543,16 @@ g:ale_set_signs                                               *g:ale_set_signs*
   To limit the number of signs ALE will set, see |g:ale_max_signs|.
 
 
+g:ale_sign_priority                                       *g:ale_sign_priority*
+
+  Type: |Number|
+  Default: `30`
+
+  From Neovim 0.4.0 and Vim 8.1, ALE can set sign priority to all signs. The
+  larger this value is, the higher priority ALE signs have over other plugin
+  signs. See |sign-priority| for further details on how priority works.
+
+
 g:ale_shell                                                       *g:ale_shell*
 
   Type: |String|