Template names with aliases for paths

I think it would be nice feature if express could handle template names with aliases for paths.

For example 
```
var aliases =  { 
    alias1: __dirname + '/templates/main/'
    alias2: __dirname + '/templates/login/partials''
  };
app.set('aliases', aliases);
...
res.render('alias1::index', {}); // will resolve to __dirname + '/templates/main/index'
res.render('alias2::index', {}); // will resolve to __dirname + '/templates/login/partials/index'
```
Note that :: is separator. It can be changed to something else with app.set('alias separator'). 
Substring before :: is alias name and substring after is normal template name