Return results as soon as we have them

[sirbrillig]

Fixes #38

This PR adds a new API (the functions search_and_format_callback and search_callback) that return nothing and instead operate via callbacks; the result callback is called for each result and the error callback is called for each error. This should make for a much simpler consumer API in most cases.

To do

• Make this optional based on config? Maybe it doesn't matter.
• Document the new API.
• Add tests for new API.
• Refactor non-callback search to use new API? It seems flexible enough to do that without needing a mutex.