Improve backup system

Improve backup system

This commit introduces several improvements to the backup system.

* Backups are made every 8 seconds for buffers that have been modified
  since the last backup.

* The `permbackup` option allows users to specify that backups should
  be kept permanently.

* `The backupdir` option allows users to store backups in a custom
   directory.

Fixes #1641
Fixes #1536
Ref #1539 (removes possibility of race condition for backups)