Support for netstandard 2.0

> Package lightbuzz-smtp 1.8.0 is not compatible with netstandard2.0 (.NETStandard,Version=v2.0). Package lightbuzz-smtp 1.8.0 supports: netcore (.NETCore,Version=v0.0)

I tried to add the current nuget package to an Azure Function (v2) Would be great if this works, because the options to send mail from Azure Functions is limited. Most samples use SendGrid as 3rd party to send mail.
