Plugin manager

Plugin manager for the new plugin structure

 - Install
  - From Rubygems
    - Latest
     - Specific version
  - From gem file

 - List installed plugins
   - All
   - Specific name
   - Specific group

 - Uninstall a plugin

 - Update
   - Single plugin to latest version
   - Single plugin to specific version
   - All to latest version
```ruby
gem_path  = (gem =~ /\.gem$/ && File.file?(gem)) ? gem : LogStash::PluginManager::Util.download_gem(gem, version)
```