Release era 0.3.1

[Joe Roe]

Prepare for release:

• Check that description is informative
• Check licensing of included files
• devtools::build_readme()
• usethis::use_cran_comments()
• devtools::check(remote = TRUE, manual = TRUE)
• devtools::check_win_devel()
• rhub::check_for_cran() *
• Update cran-comments.md
• Review pkgdown reference index for, e.g., missing topics
• Draft blog post

Submit to CRAN:

• usethis::use_version('minor')
• devtools::submit_cran()
• Approve email

Wait for CRAN...

• Accepted 🎉
• Delete CRAN-RELEASE
• usethis::use_github_release()
• usethis::use_dev_version()
• Update install instructions in README
• Finish blog post
• Tweet
• Add link to blog post in pkgdown news menu

[Joe Roe]

Response from CRAN maintainers:

  Found the following (possibly) invalid URLs:
    URL: https://www.tidyverse.org/lifecycle/#maturing (moved to
      https://lifecycle.r-lib.org/articles/stages.html)
      From: README.md
      Status: 200
      Message: OK

Will switch for a <repostatus.org> badge and resubmit as 0.3.1

[Joe Roe]

Instead of rhub::check_for_cran(), needed to use:

rhub::check_for_cran(env_vars=c(`_R_CHECK_FORCE_SUGGESTS_` = "true",
  `_R_CHECK_CRAN_INCOMING_USE_ASPELL_` = "true", R_COMPILE_AND_INSTALL_PACKAGES = "always"))

To workaround r-hub/rhub#374.