Plugins

This implements a simple plugin interface for hologram.

Plugins can be listed in the config yml like:

```
plugins:
 - my_plugin.rb
 - your_plugin.rb
```
Hologram will call these plugins in two cases:

1. Just after a doc_block has been created: `plugin.block(scope, config, comment_block, file_name)`
2. And after all comments have been processed: `plugin.finalize(scope, config, pages)`

`scope` is a dictionary that is shared between all plugins and all calls to any plugin methods. I'll have an example plug in to share hopefully in a few hours from now.