integration of send invoice

[hillelcoren]

Thanks for creating the PR!

Is there a reason you added a new class rather than add the function to the invoice model?

[TheJenne18]

To stay consistent with the routing: public static $route = 'email_invoice';

[hillelcoren]

Hmm... I think it may be better to add the method to the existing class and use the 'email_invoice' option

https://invoice-ninja.readthedocs.io/en/latest/api.html#options