Add support for adding multiple attachments to an invoice/etc mail using hooks

Currently in Sliced Invoices it is hard-coded that only one attachment can be added to an invoice/etc. e-mail.

Allowing for multiple attachments, by using hooks, would allow many additional uses for developers.. Eg. add an UBL invoice file, add your conditions PDF, etc.

It would only be a small modification to make Sliced_Notifications::get_attachments() return an (empty) array of files and add an additional hook `sliced_email_attachments` after the current `sliced_email_attachment` (watch the S at the end 🙂