Companion pull request for #1 (Adds a few alternative paths for plugins.json)

Adds the feature I requested in fgrehm/vundler#1.

What I mean by that is that certain paths such as `$REPO_ROOT/.vagrant_plugins(.json)` or `$REPO_ROOT/vagrant/plugins.json` would help better organize the plugins file. Recently I've been splitting multiVM Vagrantfiles into a vagrant directory and giving them extensions based on their purpose.
```ruby
Vagrant.configure('2') do |config|
  config.vm.box = 'precise64'
  config.vm.box_url = 'http://files.vagrantup.com/precise64.box'
end

vagrantfiles = %w[vagrant/Vagrantfile.client vagrant/Vagrantfile.server]
vagrantfiles.each do |vagrantfile|
  load File.expand_path(vagrantfile) if File.exists?(vagrantfile)
end
```

https://github.com/ecarey-r7/multiple_vagrantfiles#best-solution