Shorten the app path if possible

When I open a file with, for example, `:Emodel user`, the plugin always opens the absolute path, instead of the relative one. Adding a simple `fnamemodify` changes this.

I'm actually surprised this happens, since, if I just `edit` the absolute path myself, it's nicely shortened. Not sure why this happens. Incidentally, I also found [this function here](https://github.com/tpope/vim-rails/blob/0bb6d900af43c0fc274980ec2a8037b1f7dfdd7a/autoload/rails.vim#L3153), so maybe that's the one that should be used in `s:readable_open_command`? Or maybe the path should be relativized there?