Summary

Adds configuration for CircleCI builds. This adds a sensible linux build pipeline (hopefully without the problems Travis caused)

Details

This is just the configuration, you have to add the project at circleci.com yourself, otherwise the webhooks won't register.

if we added a new build configuration for linux/macos builds we could reduce the mess of 'dotnet build' commands, but that isn't really necessary.
this will not build the NETFX projects, although this is possible with .NET Core + Mono, it would probably require setting up a new docker image, which isn't really worth the trouble.
CircleCI build numbers won't match appveyor versions. This is intentional.
also, remember to add badge to readme afterwards, eg: https://circleci.com/gh/NaamloosDT/DSharpPlus.svg?&style=shield