Use icon from activate option bundle in notification message.

This allow user to change Terminal Notifier default icon without need to
create specific bundles. For example to use Finder icon simply run:

terminal-notifier -activate com.apple.finder -title Finder -message "Message with Finder icon"

This could please users from #31.

Signed-off-by: Sébastien Gross <seb•ɑƬ•chezwam•ɖɵʈ•org>