experimental/vim-vam-pathogen-vimrc-support

This patch implements derving a .vimrc from vim-plugins.nix loading those
plugins by either Pathogen or VAM (VAM seems to be slightly faster and is much
more powerful).

Example:

  environment.systemPackages = [
   # default plain vim
   vim_configurable

   # vim which get's called vim-with-addon-nix
   (vim_configurable.customize {
      name = "vim-with-addon-nix";
     vimrcConfig.vam.pluginDictionaries = [{name = "vim-addon-nix"; }];
   })
  ];

This way you can provide an "enhanced Vim" and a standard Vim.

Details about what this commit changes:
1) provide a new toplevel name vimrc which
  * provides a way to build up a .vimrc using either pathogen or VAM (knowing about plugin dependencies by name)
  * can enhance vim to support. vim.customize { name = "name-user"; vam.pluginDictionaries and/or pathogen.pluginNames = .. }
  * introduce rtp names for each vim plugin pointing to the runtimepath path
  * suggest naming to be the same as vim-pi so that VAM's dependencies work
  * derive some packages as example from vim-pi using VAM's new autoload/nix.vim
    supporting simple dependencies
  * test case for vim-addon-nix for VAM/pathogen

2) enhance vim_configurable to support .customize

3) update many plugins by using VAM's implementation not rewriting those which
  * vim-pi doesn't know about the git source yet (TODO: make vim-pi be aware of
    those)
  * have special build code

This commit partially conflicts with commits done by Bjørn Forsman starting by
37f96162, eg the one using lower case attr and pkg names, because they don't
match vim-pi (eg YouCompleteMe). Rather than resolving the conflict this just
adds aliases so that both names can be used