lib: add cli preprocessor for `|` actions

lib: add cli preprocessor for `|` actions

This patch adds a CLI preprocessor function that activates when `|` is
found in the command. This is the start of adding support for some text
processing utilities intended for inline use. The first one implemented
here is `| include`, which provides grep-like filtering of command
output.

Signed-off-by: Quentin Young <qlyoung@cumulusnetworks.com>