Changed View forge to allow setting filename later

[dmyers]

So you can do (new functionality):

$view = View_Twig::forge();
$view->set_filename('filename');

Or do (existing functionality):

$view = View_Twig::forge('filename');
$view2 = View::forge('filename.twig');

Doesn't conflict with default fuel core either:

$view = View::forge('errors/production');

This matches the core fuel View class more.