Add more fields to Sliced Invoices & Contact Form 7 plugin

Hello,

recently we have been configuring a website for a client and saw that the integration allows us to add sliced_client_name field, however the E-Mail-Templates of sliced invoice only allows the usage of client_first_name or client_last_name, which makes it impossible to use the name in the field in the customer E-Mail.

It would be nice to either be able to use sliced_client_name in E-Mail, or add client_first_name or client_last_name to the Contact Form 7 form.