Version 1.18.0

Relatively minor release with the following changes:

• Test on Go 1.18 and update the minimum Go and CI-tested version from 1.13 to 1.14.
• Wire up stdin in system() and don't buffer stdout in goawk command if stdout is a terminal so that you can now play Tetris with GoAWK. :-)
• Wire up ctx through system() for ExecuteContext.
• Various test and documentation improvements, including a comparison to csvkit in the CSV docs, updating CSV benchmarks to avoid huge.csv, and adding explicit tests using ASCII and Unicode unit and record separators.
• Repo cleanup: moved the awkgo directory and code to a branch and removed it in the master branch; removed the unnecessary examples directory.

See the list of commits.