Add support for ISSO (http://posativ.org/isso/) comment system

[silverhook]

ISSO seems to be a really nice lightweight Disqus alternative that you can host yourself.

@oerdnj has already managed to get it working fine with Pelican and submitted a pull request that implements it into the main tree, but it seems it should fit better here instead.

getpelican/pelican#1253

[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