implement a plugin framework

[mmomtchev]

here is my first draft

A plugin has to export:

export async function init();
export function parse(file, config, api);
export function shallow(file);

I have included a small mock_plugin.js for unit testing

[mmomtchev]

#1441

[mmomtchev]

You can also take a look at a working plugin:
https://github.com/mmomtchev/documentation-polyglot

[mmomtchev]

@anthony-redFox w8, I just found a major problem - expandInputs happens before globbing so there is no way to have some files use shallow dependencies and others - full tracking
Unless you have an idea?