Add "email an invoice" method
Add "email an invoice" method

| Q             | A
| ------------- | ---
| Branch        | master
| Bug fix?      | no
| New feature?  | yes
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | n/a
| License       | MIT
| Doc PR        | n/a

See: https://www.zoho.com/books/api/v3/invoices/#email-an-invoice

opsway/zohobooks-api