[os] Make `vim_plugins` also update plugins
[os] Make `vim_plugins` also update plugins

Improve `vim_plugins.sh` script so that it not only installs the
specified Vim plugins, but also updates the ones that are already
installed.

oddlord/dotfiles,girishramnani/dotfiles,b-boogaard/dotfiles,zlot/dotfiles,mickael83/ubuntu-dotfiles,christophermoura/dotfiles,makabde/dotfiles,zlot/dotfiles,makabde/dotfiles,christophermoura/dotfiles,newtonne/dotfiles,AlbertoElias/dotfiles,AlbertoElias/dotfiles,newtonne/dotfiles,mickael83/ubuntu-dotfiles,EDiLD/dotfiles,girishramnani/dotfiles,pbrooks/dotfiles,EDiLD/dotfiles