Allow custom javascript in posts

[turtlemonvh]

See, for example:

https://notabug.org/jorgesumle/pelican-js
https://github.com/mortada/pelican_javascript

A few options for doing this

1. Integrate one of these plugins directly (native support)
2. Set up flex so that you can extend certain sections of the base template with included templates (e.g. so the user can drop an extra_js.html to a certain path on disk and that gets injected into the base template at run time if it exists).

Number 2 has the benefit of making it so users can install a variety of plugins that may ordinarily require them to modify the base template, but instead they can change the structure of the page by simply dropping a template fragment into a directory.

Let me know if this sounds interesting. I am happy to put together a PR with a working implementation.

[alexandrevicenzi]

@turtlemonvh It could be done, perhaps with the plugin, we already use some.