Allow for browser-extension plugins

It would be really nice to augment the functionality of the browser extension through plugins and well defined extension points. Annotator.js offers plugins ... perhaps the hypothesis extension can expose this as well as extension points for actions, adding new functionality via buttons and hooks etc. This could help third parties rapidly integrate Hypothesis with existing applications.

Forgive me if this is already being considered. I have not found anything on it yet. The thought occurred to me while wishing for GitHub Projects/Issues integration with Hypothesis.