feat: plugins?..

I would like to introduce the idea of a plug in system in zola... Some like it, some don't, but it allows for a more decentralized life of the project, letting people develop different things for different use cases instead of having to open a million prs/issues for the maintainers to read.

#3157 could easily be a well implemented plug in, giving zola the ability to infer dates from git without having to implement it in the main source code.

Just a thought, plug in systems don't belong everywhere, but it is pretty common for other ssgs to do as far as I've seen. (Eleventy, Hugo modules, Gatsby plugins, or WordPress plugins)