* add a ~/.persistent-shell-history.yaml
* move the db from gdbm to sqlite
 - tables
  ** hostname
   -- id
   -- name
  ** command
   -- id
   -- value
  ** record
   -- id
   -- timestamp
   -- hostname_id
   -- command_id
* migration from current Marshal version need only to check the value, for if
  it starts with "\x04\b{"