Add doc_comment to test README.md

[nicochatzi]

Another one from a suggestion by @Boscop. This adds README's Rust code block as a doctest in lib.rs. doc_comment seems less intrusive than skeptic since it doesn't require a separate build step that generates a temporary file and paste it into a test file. It does have the caveat that if the test fails it looks as if the lib.rs failed. To make it more obvious that a test failure in lib is related to the README, this PR moves plugin_main code into a separate file. Any thoughts?