Proposal: IEx prompt functions

[Anderson Cook]

Currently IEx allows configuration of various prompts as strings, I was curious if we wanted to allow configured nullary functions to return a string instead. This would allow users to configure their own function to add things like timestamps to their prompts.

ex:
```elixir
config :iex, default_prompt: fn -> "#{DateTime.to_string(DateTime.utc_now())} %prefix(%counter)" end
```

I'd also be interested in adding timing data between executions to the prompt, but that would require a different change.