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
I see this
```ruby
spec.metadata['logstash_plugin'] == 'true'
```
repeated in a few places. I would suggest writing a method such as 

```ruby
def is_logstash_plugin?(spec)
  spec.metadata['logstash_plugin'] == 'true'
end
``` 

and use this. the benefit is that if we ever change the way we identify plugins, only one method will need to be updated, also it is much more readable.