promote more CLI things to API

[mcandre]

• DIRECTORY_EXCLUSIONS
• Recursive directory walking

[mcandre]

Toyed with DIRECTORY_EXCLUSIONS in the API. That is doable.

However, recursive directory walking would take some more work.

Ideally, we stream findings (via channels), rather than synchronously aggregating them.

Ideally, we provide a separate channel for debug findings, versus error and warning findings.

https://doc.rust-lang.org/rust-by-example/std_misc/channels.html

We would also want to test how the application responds to SIGINT.