Add a new lightweight way to create plugins

I'm proposing that we add a method to Ink that lets people create a new plugin with only a hash. Here's what that looks like:

```ruby
require 'octopress-ink'

Octopress::Ink.new_plugin({
  name:        'Awesome Sauce',
  slug:        'awesome-sauce',
  assets_path: File.expand_path(File.dirname(__FILE__)),
  description: "Test some plugins y'all"
})
```

Currently people must subclass `Octopress::Ink::Plugin` and then add a method that returns a hash. If this pull request is merged, the only folks who do that will be those who need to override methods of the plugin class. For the large majority of folks, that won't be necessary.

What do you think @parkr?