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
Since .any? takes a block, you can do simply:

```ruby
>> Gem::Specification.any? { |x| x.name == "bundler" }
=> true
>> Gem::Specification.any? { |x| x.name == "whoa" }
=> false
```