Iterate on keys pressed instead of chars

Only the unix implementation works.
The windows impl does not even compile...

Parametrize Editor by the Completer type

This allows the `Editor` to implement `Sync` as long as it contains a completer which implement `Sync`. It lets the `Editor` take ownership of the completer.

And do not ignorespace by default.
And do not panic when the history is empty

Make History ignore space/dups configurable (#50)

It seems that we need both (out and output_handle)