Adding a complete plugin system

(Followup to #683)

After reading [this part](https://github.com/documentationjs/documentation/blob/master/docs/FAQ.md#why-use-documentation-instead-of-jsdoc) of the FAQ, I thought that documentationjs had a "well documented and flexible" plugin system because it said `Customization points like plugins & templates are well documented and flexible`.
What I expect from a plugin is to be able to fully alter any step of the generation process. From the config reading to after the files have been written. (As can provide [esdoc with its 10 callbacks](https://esdoc.org/manual/api.html) and [jsdoc with its 9 events](http://usejsdoc.org/about-plugins.html).)

Here is a very concrete example:
I am currently working on a NodeJS web server. I have a lot of classes, functions, etc that are documented using jsdoc style comments but I would also like to have a custom `@route {GET} /users` tag that would attach specific informations to the AST.
To achieve this, I would "simply" create a plugin, make it listen for some kind of `onDocTag` event where `tag === "route"` and attach some additional properties to the doc node:
```js
{
kind: "route",
method: "GET",
path: "/users"
}
```

Then, later in my plugin, I would be able to alter the generated output by adding some kind of "meta format tag" in order to have specific formatting of my routes.

I hope this use case is clear enough because it's a very real thing I'd like to be able to do.

(I am not looking for a workaround, I'd really like to have custom tags handled by the plugin defining them)