Ability to run EXPLAIN ... on a query

It would be super handy when looking at a hairy query to be able to click a link to feed that query into an EXPLAIN and get the result, to see what the planner does. That'd involve the plugin knowing how to get a DB handle to use though, and would fall down if the app uses multiple DBs. Hmm.

Start by reading Catalyst::Plugin::Profile::DBI::Log and the DBI::Log query-capture flow to understand what query data and database handles are available. Clarify how a user-triggered EXPLAIN should work when an application uses multiple databases; done should include a defined link action and returned planner result.

Written by the indexing model from the issue text.