Add shell_pipe plugin as submodule

[rwev]

https://github.com/rwev/shell_pipe

[kdeldycke]

This mono-repository no longer accept any submission of Pelican plugin as submodules. Instead, new plugin should be packaged as independent modules so you'll be free to publish them out on PyPi.

To help, there a plugin template available at: https://github.com/getpelican/cookiecutter-pelican-plugin