Define guidelines for core plugins

[sk-]

There are a bunch of PRs with new plugin proposals. However it is not clear which of those should be accepted.

We should have a clear policy regarding which packages can make it into the core. And add support for external plugins if that's not already supported.

For those plugins that make it into core we should require:

• comprehensive testing
• reduction in size of at least X% in a benchmark dataset.

Also, at the very least I'd halt accepting plugins until we have:

• validation testing
• codebase is more stable/cleaned up

[TrySound]

I don't accept plugins yet. Will make svgo more reliable first.