archivers/plakar: Add new port

plakar is a free and opensource utility to create distributed, versioned
backups with compression, encryption and data deduplication.

With a very clear goal to simplify backups, it is designed to be
straightforward to use.
It requires a few seconds to install and only a few minutes to get
backups set up and running: creating a backup is as simple as typing
plakar backup and restoring as simple as typing plakar restore.