fix(cache): default `clean: true` when necessary, add `extraCacheKeys` option

[oscard0m]

Summary

• [x] feat(cache): set options.clean as 'true' by default
• [x] feat(index): add 'extraCacheKeys' option
• [x] fix(cache): apply PR comment fixes

Context

Fixes #228

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Questions

• Do you know if this approach is correct? It has been a long time since I do not touch rollup and this plugin, so I want to validate this step before getting deeper.

unless the user provided the extraCacheKeys option, we then leave everything up to the user

• Where is the extraCacheKeys option declared? I don't find it. (_originally posted here)

I believe the only two are sourceMapCallback and transformers

• Does this mean I need to do some logic to decide the default value for the clean option depending on these 2 other options?

[oscard0m]

Applied more changes to the draft.

Now it's not clear to me which part of the code (and how) the extraCacheKeys should be treated.