[FEATURE REQUEST] Extensions support

[silvioprog]

One of the most important features in the popular servers like Nginx and Apache is the ability to extend them. For example:

• List of Nginx extensions: https://www.nginx.com/resources/wiki/modules
• List of Apache modules: https://en.wikipedia.org/wiki/List_of_Apache_modules

It is known that, due to the complexity, it isn't so easy to implement it in see, however, such feature would allow the see tool to get a larger community, since any users would be able to develop their own extensions.