Add --notes option to edit command

[ngsilverman]

• Makes it possible to edit the notes of an existing reminder.
• Edit the title, or notes, or both at the same time (basically behaves as can reasonably be expected imo).
• I thought about making it possible to append to the existing notes but replacing it is more flexible and consistent with the way the title can already be edited.

This is a small part of what was requested on #59.

[keith]

thanks!