'keywordprg': support ex commands

- new feature: if the first character of 'keywordprg' is ":", the
  command is invoked as a Vim ex-command prefixed with [count].
- change default 'keywordprg' to :Man

src/nvim/options.lua

@@ -1329,7 +1329,7 @@ return {
       expand=true,
       varname='p_kp',
       defaults={
-        if_true={vi="man -s"},
+        if_true={vi=":Man"},
       }
     },
     {