Added new --unsafe mode: -k cleartext

Using the 'cleartext' name for a key will read it from stdin and
expect it to be unencrypted. This is an unsafe mode of operation,
but useful i.e. to open remote tombs by piping the key over ssh,
or using a different software than Tomb to de/crypt keys.

This commit also renames the flag --unsecure-dev-mode into --unsafe
and includes a minor fix to the last opened date shown on open.

Addressing issue #161