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 think we should call it "PLUGIN" even though the underlying implementation is gem.

Also, you can omit the `attribute_name` setting because it defaults to the lowercased parameter name. Same for flags:

```
class X < Clamp::Command
  parameter "FANCY", "Some fancy word."
  option "--attire", "CLOTHING", "What attire?"

  def execute
    puts "Attire: #{attire}"
    puts "Fancy: #{fancy}"
  end
end
```

Running:
```
% ruby test.rb --attire pants green
Attire: pants
Fancy: green
```

(Summary: Attribute names are sanely chosen by default. No need to specify the attribute name always)