allow -c to be used with explicit file

ldoc.lua

@@ -284,6 +284,13 @@ if args.file == '.' then
       args.file = abspath(args.file)
    end
 else
+   -- user-provided config file
+   if args.config ~= 'config.ld' then
+      local err
+      config_dir,err = read_ldoc_config(args.config)
+      if err then quit("no "..quote(args.config).." found") end
+   end
+   -- with user-provided file
    args.file = abspath(args.file)
 end
 

tests/styles/config/config.ld

@@ -0,0 +1,2 @@
+no_return_or_parms=true
+no_summary=true