Add a configuration entry where plugins can add a function to be called at template's context calculation time

My plugin renders template data to json for later consuming on the client side. The plugin work in tandem with a modified bootstrap3-jinja theme which uses that same "static post data" to render. This is because fragments of the templates are shared with the JS code to render them as a single page app instead of reloading all the page. Currently a work in progress.

See https://github.com/azazel75/plugins/blob/master/v7/spa/spa.py
and  https://github.com/azazel75/nikola-themes/tree/master/v7/spa-jinja/templates