[O] org-timer-set-timer org-notify message and sound arguments

[Devin Homan]

The current implementation of org-timer-set-timer calls org-notify with
the PLAY-SOUND argument set to 't', which gets passed to
org-clock-play-sound that then in turn calls the beep function. The
value of this argument should be defined in a defvar so as to allow the
user to modify or dynamically set its value. Additionally, a similar
behaviour should be allowed for the message by having the first cond
case, in the cond that sets hl, return the value of a defvar should that
defvar be a string type.

Sincerely,

Devin