Feat/add support for mix

[viglioni]

Motivation

Use hex libraries within code blocks on org-mode

What was done

• ob-elixir-ensure-session checks if there is a mix.exs file in the same directory as the org file, if yes it uses the iex -S mix command instead of iex.
• added how to use it on readme.org
• aligned part of the code using emacs align command