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)

cmd/micro/micro.go

@@ -274,7 +274,7 @@ func main() {
 			fmt.Println("Micro encountered an error:", err)
 			// backup all open buffers
 			for _, b := range buffer.OpenBuffers {
-				b.Backup(false)
+				b.Backup()
 			}
 			// Print the stack trace too
 			fmt.Print(errors.Wrap(err, 2).ErrorStack())