Allow querying the contexts in the HTML report

[gaborbernat]

Would be nice to have a page where one could enter the context prefix and would return a search result that link to the files that has that context 🤔

[nedbat]

This is possible by querying the SQLite database directly:

% sqlite3 .coverage "select distinct f.path from file f, arc a, context c where f.id = a.file_id and a.context_id = c.id and c.context like 'prefix%';"

I'm wondering about other ways to make the information available. There's a coverage debug data command, but coverage debug contexts doesn't sound like what you would want.