Add "hcl append" command for adding additional notes to currently running timer.

Have a long-running timer for a long-running task that you want to keep a running log of notes for?  Let me introduce you to `hcl append`.

Fire off `hcl append "Your new note."` and it'll tack on a new paragraph (_i.e._, after two `\n`) to your notes field for your currently running timer.

Inspired by @wycats feature request at dinner last night.