vim_configurable and neovim have both supported a mechanism to build
them with a custom vimrc that supports plugins from Nix. This updates
MacVim to support the same sort of configuration using an expression
like

  macvim.configure {
    customRC = ''
      # custom configuration goes here
    '';
    packages.myVimPackage = with pkgs.vimPlugins; {
      start = [ youcompleteme fugitive ];
      opt = [ phpCompletion elm-vim ];
    }
  }

Once configured, .override will allow for editing the configuration.

Like vim_configurable and neovim, configuring macvim does not require
rebuilding it. Also like them, configuring macvim turns off the user
vimrc file.

macvim: add configuration similar to vim_configurable and neovim