New maintainer + a bunch of maintenance revisions

[kbroman]

• New maintainer Karl Broman; see Issue #33
• Changed license, CC0 -> MIT
• Fixed a bunch of small changes, mostly regarding changes in tidyverse
• dropped data.table dependence; can use dplyr::bind_rows() instead of data.table::rbindlist(). See Issue #24
• switched from appveyor/travis to Github Actions for automated package checks
• include testthat tests when running R CMD check
• Change examples from \dontrun{} to \donttest{}

The package was archived on CRAN, but I think it could now be restored.