[Ml-yokadi] Note support

[Aurélien Gâteau]

Hi,

I just pushed my note branch to github. It is still unfinished, but I'd 
like to get some feedback on it.

What is a note? A note is reference information you want to attach to a 
project. It is not a task: it does not need to be done. Until now I have 
been creating notes by creating tasks with the @note keyword. This works 
reasonably well, but it results in the output of t_list being cluttered 
with rows which are not actionable items.

I built on this to implement note support: internally a note is a task 
with the @note keyword (will probably move to @_note before release). 
This means it is easy to turn a task into a note and vice-versa.

I added two new commands:
- n_add creates a note (ie, a task with the @note keyword).
- n_list lists only notes.

I also modified t_list to skip tasks with the @note keyword.

What do you think of this?

Aurélien