Add MixpanelVapor

[petrpavlik]

• Project Name: MixpanelVapor
• Project URL: https://github.com/petrpavlik/MixpanelVapor
• Project Description: Mixpanel SDK intended for server-side Swift apps using Vapor
• Why it should be added to awesome-vapor:
• Project has at least 15 stars (if it’s a GitHub project)
• Project supports at least Vapor 3
• Project supports at least Swift 5
• I used correctly sized dash and ended the entry with a full stop 🤓