Broaden scope of suppressed warnings for listings without a main fn
Broaden scope of suppressed warnings for listings without a main fn

At present, code listings without a main function will be wrapped in one and
annotated with an allow lint check attribute provided by the following [code][]:

```
format!(
    "\n# #![allow(unused_variables)]\n{}#fn main() {{\n{}#}}",
    attrs, code
)
```

A broader lint check attribute such as `#![allow(unused)]` seems like it might
better fit the apparent intent of this code.

Addresses: https://github.com/rust-lang/mdBook/issues/1192

[code]: https://github.com/rust-lang/mdBook/blob/769cc0a7c14fe133fc5eb223f435473730d0086e/src/renderer/html_handlebars/hbs_renderer.rs#L635-L638

azerupi/mdBook,azerupi/mdBook,azerupi/mdBook,azerupi/mdBook