mlr 2.18.0

## Cran Repository Policy

- [x] Reviewed CRP last edited 2020-07-11.

## R CMD check results

- [x] Checked locally, R 4.0.2
- [x] Checked on CI system, R 4.0.2
- [x] Checked on win-builder, R devel

Check the boxes above after successful execution and remove this line. Then run `fledge::release()`.

## Current CRAN check results

- [x] Checked on 2020-09-30, problems found: https://cran.r-project.org/web/checks/check_results_mlr.html
- [x] NOTE: r-devel-linux-x86_64-debian-clang, r-devel-linux-x86_64-debian-gcc, r-patched-linux-x86_64, r-release-linux-x86_64
- [x] NOTE: r-devel-linux-x86_64-debian-clang, r-devel-linux-x86_64-debian-gcc, r-patched-linux-x86_64, r-release-linux-x86_64
- [x] NOTE: r-devel-linux-x86_64-fedora-clang, r-patched-solaris-x86, r-release-macos-x86_64, r-release-windows-ix86+x86_64, r-oldrel-macos-x86_64
- [x] NOTE: r-patched-solaris-x86

Check results at: https://cran.r-project.org/web/checks/check_results_mlr.html
