Explore the composability of filters

[dgkf]

Ex. the "medium risk" filter should be the "low risk" plus additional conditions. How can we support this style of composability.

[llrs-roche]

From #5:

I'd still love to see composable filters, ie

cooldown <- package_filter(as.Date(Published) > Sys.Date() - 14)
good_docs <- package_filter(...)
established_w_good_docs <- package_filter(cooldown() & good_docs())

but I think that's a bigger lift than we can manage with this PR.

I agree this would be a nice idea. I also like the operators for this kind of composition, but we need to think well how are they joint and what is allowed.
Would be allow to exclude them? ie good_docs() & !high_risk()?