Add support for multi-line comments

[afjoseph]

Closes #17

Hey guys. I actually just stumbled upon Taskbook. The codebase is very clean and looks like a great project. Kudos to everyone here!

This PR will add "comments" to a task. I use TB for my daily task management and many times, I would like to make an "update" to a task. TaskWarrior calls these "Annotations".

Demonstration

[0] % tb -t @daily This is my new task

 ✔  Created task: 6

[0] % tb -z 6

# Opens my default editor and wait until it exits

 ✔  Commented on task: 6

[0] % tb

  @daily [0/1]
    6. ☐  This is my new task
       - I :heart: Taskbook
         - This supports multiline comments...
           - without an issue

  0% of all tasks complete.
  0 done · 0 in-progress · 3 pending · 0 notes

Disclaimer

I am, by no means, a JS developer. Please lemme know if the commit could be cleaned further and I'll take care of it.

Cheers and thank you for the project