Backups containing whole todolist and archive can be now saved after
execution of each "read-write" command. Furthermore this change creates
base for eventual "revert" command.

Backups are safely stored and indexed in our own JSON-based format which
is compatible with python2.x and python3.x. We also use zlib compression
to minimize size of backup file. Path of the backup file is always
relative to the todo file, so backups from different todo files won't
mix up.

User can configure number of stored backups with new config option -
"backup_count". Any positive number will tell topydo to store that very
number of backups. Setting "backup_count" to 0 will completely turn off
backup functionality.