add options for each engine. extracted from Onebox.options based on class name

allow each engine to access its own options 
For example to set options for the engine github_blob_onebox.rb 

```ruby
Onebox.options = {
  GithubBlobOnebox => {:SHOW_LINE_NUMBER => true}
}
```

then from within the engine options can be accessed 
```ruby
#get the engine option
self.options[:SHOW_LINE_NUMBER]

```