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
Sadly windows has trouble with a second `cargo_process` (dunno why) because it blows away the directory and rebuilds it. You may want to replace this with `p.process(cargo_dir().join("cargo")).arg("run")`