Add user setting to auto-format code blocks on eval and on save

[zachallaun]

While this is available through keyboard-shortcut with shift+alt+f, I'd love options to auto-format Elixir code when a cell is evaluated as well as auto-format all Elixir cells when the notebook is saved.

This sequence of events is extremely common for me:

• Write or edit some code
• Eval it with shift+enter to go to next block
• Realize the code block isn't properly formatted, so go back up with k
• You can't use the shift+alt+f shortcut unless you're in insert mode, so press enter, then shift+alt+f, then shift+enter again so that the * goes away next to "evaluated" 🙂

[zachallaun]

Decided not to implement due to some confounding factors after discussion in #2077.