Add support for .iex files

This is the first half of implementing #1062.

In the description of .iex files I mention the `f()` helper which would erase variable bindings. IEx does not have it yet, but it would be really useful as the last expression in an .iex file if the user doesn't want to keep var bindings. Or, perhaps, there is another way to scope the code in the .iex file lexically so it doesn't leak into the shell if the user doesn't want it to?

The fact that it leaks by default is by design.