Skip Doctests When Running cargo test

I don't use doctests when I'm working on small, one-off Rust apps. The default functionality of `cargo test`` it so run them. The result is an empty report that shows up last. It's no big deal but it throws me off seeing `0 passed`` in the report.

I use this in my `cargo.toml`` file to turn off the tests and the subsequent report:

doctest = false

It's a nice little quality of live improvement.