Add `cargo run --example` and `cargo run --bin`

This lets users run any executable from the `examples` or `bin` directories by
passing its name with `--example` or `--bin` flag.

If neither is specified, we fall back to the old behaviour (running the only bin
target in the project, failing if there are more).

Closes #538
The match is actually a sub-expression, so you may want to have `name_eq && match env { ... }` to ensure it's on all cases