PluginManager interface (2 of 3)

[unode]

This is an attempt to split #226 into smaller PRs, easier to review. This PR should be merged after #323 and before #226.

Most glue code is present in this PR.

[unode]

I'll try to merge this and #326 and address test failures after as there's a little too much in flux.

[codeclimate[bot]]

Code Climate has analyzed commit f14b7130 and detected 2 issues on this pull request.

Here's the issue category breakdown:

Category	Count
Complexity	2

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