Add persistent history to iex (ie ~/.iex_history)

First off... :heart_eyes_cat: thanks for creating Elixir!

In the course of learning it, I miss not having a record of iex command entries.  So I added that feature :smiley_cat:.

Use case:
Having a persistent ~/.iex_history between sessions is helpful when actively using a repl for development.  It also allows replaying prior histories by using TMUX, Vim, and a plugin to pipe text into a live iex session.

Note: It's a private method so I'm not sure about the standard practice for testing, but please feel free to nudge me in the right direction.

:smile:

-Zander