Ability to search pages by CSS selector

[chosak]

It would be nice to be able to search page HTML that contains elements matching a given CSS selector.

To do this in a performant way that is consistent with other search methods, we would want to implement this as a SELECT in the database somehow.

For the current SQLite backend, we might be able to use sqlite-html (unfortunately requiring a newer Python version than other existing requirement wpull supports). This would also require compiling Python with --enable-loadable-sqlite-extensions.

Looking ahead to a possible Kubernetes deployment that uses PostgreSQL RDS instead, we might be able to use PLV8 to execute the query using JavaScript. PLV8 is available as an extension in AWS RDS.

(As discussed with @anselmbradford and @wpears)