Add EOF handler

This pull req is related to https://github.com/lestrrat/peco/pull/55 a bit.
Unix shell handles C-d as EOF by default, and it behaves like:

- when shell has some input: delete one character forward
- when shell has no input: exit process

This behavior is useful because you can support both of exiting and forward deletion in one key.
But percol does not support this, so I think this feature should be optional.