Implement crate_description!, crate_name! and app_from_crate! macros

[nabijaczleweli]

Includes documentation.

[coveralls]

Coverage decreased (-0.2%) to 91.43% when pulling 4d9a82d on nabijaczleweli:feat/778-simple-cargo-app into 805b9b6 on kbknapp:master.

[coveralls]

Coverage decreased (-0.2%) to 91.43% when pulling 4d9a82d on nabijaczleweli:feat/778-simple-cargo-app into 805b9b6 on kbknapp:master.

[kbknapp]

Awesome, thanks for knocking this out! @homu r+

[homu]

📌 Commit 4d9a82d has been approved by kbknapp

[homu]

⚡ Test exempted - status

[Arnavion]

So

let app = clap_app! { name =>
    (author: crate_authors!())
    (version: crate_version!())
    ...
};

becomes

let app = clap_app! {
    @app (app_from_crate!())
    ...
};

right? Might be nice to document it since the only way I found out I had to put @app and the parentheses around app_from_crate!() was by reading the source for how clap_app! expands.