Use clap in some examples

[mxgrey]

clap is a popular crate that makes it extremely easy to write CLI programs in Rust.

We should include some examples that use it, for users to reference. It would also be good to verify that the way Context::default_from_env() works is compatible with clap.

Maintainers can feel free to assign this ticket to me; I'm opening this mostly as a reminder to myself to do this.

[styczen]

@mxgrey This issue should be moved to new repository with examples (https://github.com/ros2-rust/examples)

[esteve]

@mxgrey I've transfered this issue to the examples repo. Thanks @styczen for the heads-up