Schnack plugin system

[gka]

with more and more auth providers being useful to different users it might make sense to think about a simple plugin architecture for Schnack. the main purpose would be to add different authentication strategies (including anonymous), but it could also be used for other things potentially.

Plugin architecture goals v1

• dead-simple API for adding more auth providers
• ideally without messing with the database (no schema changes)
• plugins should be published in different npm packages, e.g. schnack-plugin-twitter or schnack-plugin-vk

[gka]

Other uses for plugins:

• notifications (slack, email, etc)
• importer plugins for importing comments from different sources