add configurable smtp greeter

[cgroschupp]

No description provided.

[phires]

Hi, thank you for your contribution!
Could you additionally provide an example for the configuration file?
Just adding it like so "greeter_template": "220 {{ .Hostname }} SMTP Guerrilla({{ .Version }}) {{ .Time }}"
results in an json parsing error (error="could not read config file: could not parse config file: json: cannot unmarshal array into Go struct field ServerConfig.servers.greeter_template of type template.Template")