adding the ability to set a custom notebook to send highlights to

In the .cfg file, adding "NOTEBOOK=some notebook" will add "@some notebook" to the email subject after the note's title, but before the " +"

If the NOTEBOOK field is missing completely or blank on the right side of the "=", then nothing extra will be added to the subject, and it will continue to send to the default notebook (" +" will still be added, so it will still append to the notes).