Support email headers property and simplify logic

[mousetraps]

Headers are defined as in meteor docs:
https://docs.meteor.com/api/email.html

This change enables you to set additional properties from VulcanEmail.buildAndSend, like "sender"

Tested in both simulation and non-simulation modes by setting the "sender" header with mailgun.

{
  emailName: 'test',
  ...
  headers: {
    sender: 'User Name <user@mydomain.com>'
  }
}

[SachaG]

Oh thanks! And thanks for updating both ways of calling the function.