Make `vim` load a saved session if there's a Session.vim file in the current directory.

I autoload this from .zshrc so that when I have a (https://github.com/tpope/vim-obsession)[saved session] that opens when I simply type `vim`, otherwise an empty session would open and I know that I need to initialize a session and start loading buffers. Of course if I want to open a specific file without loading the session I can use the standard vim syntax and it all works.