add options to adjust outfile variable grouping

[pekasen]

Right now, tegracli writes posts with the following file-pattern: ${user.id}.jsonl, but other formats are more disreable, e.g. ${created_at:"%Y-%m-%d"}.

[pekasen]

After much thought: we are not able to group output files by date easily, as we rely on single-user files in each group for getting the last message and the message stream is again not easily accessible from the point in the program where we want to track the latest message id.