Figure out how to make plugins more maintainable and easier to write

[ohemorange]

Proposal available here: https://docs.google.com/document/d/1bKmiu8CpsCntmJ-10CLV4UvxAaJ6qHgdGM3GivK8f2k/edit?usp=sharing

Tangentially related to our packaging discussion, since figuring out how third party plugin devs can integrate their plugins with Certbot is one of the concerns of that project.

[bmw]

Assigning myself to this issue to add my own comments since the initial discussion seems to have died down a couple weeks ago.

[LubosKolouch]

This is now quite old issue - any progress?