Implement --only-new option

[abul4fia]

This option downloads only messages with a timestamp more recent than the last one downloaded.

Implementing this feature should not be difficult. Teletolo can store the timestamp of the last successful run in the filesystem. The file should contain a list of key-value pairs, where the key is the ID of a channel and the value is the timestamp of the last access to that channel. This would allow for a nice integration with feature #4. However, this is also required even if that feature is not implemented, since each time Teletolo is run, it can be used to pull messages from different channels.