asciicast 2.3.0

• record() has a new argument: show_output, to show the output from the
  subprocess in real time.

• New SVG theme: readme, to be used in README.Rmd (and similar) files on
  GitHub. It generates two SVG files, one for light and one for dark mode,
  and creates a README.md file that automatically uses the correct one
  when viewed on GitHub.

• New SVG theme: github-dark.