feature: add option to prompt user for session goal

[h0uter]

I see that currently the active file name is appended to the session log. Instead, I find it helps me if I write down exactly what I want to achieve during the Pomodoro session in one sentence before I start it. Can you make an option to prompt the user for a session goal, and then append that session goal to the log? This would make it so I don't have to create a file for each session goal.

example session goal: "write 200 words for thesis introduction"

after the Pomodoro session is completed it would then append the following to the log note: 🍅 Friday, January 14 2022, 00:05: write 200 words for thesis introduction