Option to upgrade only CRAN packages in install_github

[mem48]

A feature request:

When using install_github() if upgrade = "ask" you are given the option to upgrade.

1: All                           
2: CRAN packages only            
3: None  
4+: specific packages

But for automating the process the upgrade can only be "default", "ask", "always", or "never", so it is not possible to choose CRAN packages only . Could a "CRAN" option be added to upgrade to allow this?

[GegznaV]

It would be a really usefull option.