Refactor core to use a plugin architecture

[kefranabg]

Split readme-md-generator into multiple packages ang manage them with yarn workspaces. Each question would be a plugin that you can easily register.

[JuroOravec]

+1. It would be great if users could select which plugins/questions to include, or could easily create their own plugins (questions), and use them.

Use cases:

1. I'd like to add a bunch of badges to my README. I use my own template, which is just the default template including the badges. Most of the badges need just username and repo name, so those are fine as those variables are already available in to the template.

   But e.g. CodeClimate has a custom ID to fetch the correct badge. So I'd like to add a question that asks for the CodeClimate badge ID, so I could use that variable in the template.

   For badges especially, the plugin architecture would be very useful as the number of different badges and their providers can change relatively fast, so it would be better if it could be managed by the community rather than the core package.

2. I'd also like to link to a CHANGELOG.md from README. So I'd also like to add a question that asks me about the URL to the CHANGELOG, so I could, again, sue that link in the template.