Refactor variables: Keep order of declaration

This shouldn't have any behavior changes for now. This is a code
refactor that should allow us to do further improvements on how
variables are handled, specially regarding respecting the declaration
order in Taskfiles, which should make it easier for the users.

Initial work on #218

cmd/task/task.go

@@ -140,9 +140,7 @@ func main() {
 	}
 
 	calls, globals := args.Parse(pflag.Args()...)
-	for name, value := range globals {
-		e.Taskfile.Vars[name] = value
-	}
+	e.Taskfile.Vars.Merge(globals)
 
 	ctx := context.Background()
 	if !watch {