Enabling customized notebook config

[bcombourieu]

Adding the possibility to add some customizable options into Notebook configuration.

Usage example (in attributes/default.rb)

# Additionnal configuration 
default[:ipynb][:NotebookApp][:additional_config] = <<-eos 
c.IPKernelApp.exec_files = [ 
    "#{File.join(default[:ipynb][:home_dir], "my_fancy_lib"}"
] 
eos 

[rgbkrk]

Makes sense. I'll leave this up to @JasonBoyles and @brint.