Add 'exec' option for custom notifiers

Pass in an 'exec' option to `growl()` to use a custom command for
notifications. It will append the message to the command unless you
include a `%s` in the command string which is expanded to the message.
'title' is expanded to a `Title: ` prefix in the quoted message.

Also `%%s` will NOT expand but `%s` will (so it can be escaped if
needed.

The hack isn't as clean as I'd like but it's kinda the way the original
data structure was designed. Perhaps this one commit could segway to a
more expressive API then it is at the moment.

Closes #45
Resolves #41

Signed-off-by: Joshua Appelman <jappelman@xebia.com>