Introduce a Configurer trait

Reuse undo::Changeset to retrieve last inserted text.
Introduce Cmd::Replace(Movement, Option<String>).
Refactor Cmd::Kill handling.

Do not catch interrupted error when SIGWINCH signal is received

Not tested

Not tested

Ensure 0 previous rows are removed when calling refresh on a freshly
created State. Before, 0+ rows were removed on an initial State
refresh. If the prompt had a newline in it, a positive number of rows
were removed, clearing past output in stdout.

Fix #109

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