ci: github actions

[mrustl]

https://github.com/r-lib/actions

E.g. used by "pkgdown" as CI (instead of Travis and Appveyor).

Pricing details: https://github.com/features/actions#pricing-details

We have 3000 minutes included in our KWB-R team plans.

[mrustl]

Testing github-actions for "private" R package kwb.package KWB-R/kwb.package@1e7aea7 (https://github.com/KWB-R/kwb.package/actions) and already spent 10% of 3000 build minutes / months within a few hours.

[mrustl]

Also discussed in #78 and should be closed by #79