Add list argument to CLI talk command

[Project proposal](https://docs.google.com/document/d/1_-vzW36GFCA23Wm415Xyf7B2DNM_-I6DFc6i4s5zGnc/edit#)

- [x] List existing talk titles
- [x] List other relevant fields: ID, speaker, event
- [x] Format list printing
- [x] Add tabulate to requirements.txt
- [x] Handles empty talk table case
- [x] Use column names to print table headers
- [x] Clean up and squash