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

To set options for the engine github_blob_onebox.rb 
Onebox.options = {
  GithubBlobOnebox => {:SHOW_LINE_NUMBER => true}
}

then from within the engine instance engine options can be accessed 
self.options[:SHOW_LINE_NUMBER]
or get all options using
Onebox.options
Merge pull request #237 from lidlanca/patch-4

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