Warn when using pipes in commands

[mcandre]

By default, sh may not respect exit codes of commands leading up to the final pipe command.

An explicit set -o pipefail may be required. However, that is unfortunately not currently supported by POSIX sh.

Encourage explicitly performing set -o pipefail && prior to each piping rule command, or moving the complex command to a dedicated shell script.

Take care to avoid false positives, when the pipe symbol is passed in a double or single quoted command argument. Perhaps we keep the scan simple and ignore the false negative of an escaped pipe string literal.