Integrate Sendy API for newsletters

[NicoJuicy]

Follow up on https://github.com/timbjames/Sendy.Net for #51 for integrating Sendy.

What it should the package do, if it doesn't: fork or recreate?

• Create HTML
• Send initial subscriber list + update
  -- How does Sendy handle unsubscribed users, which i send again?

[NicoJuicy]

Decaprecated because of #51