Mark macros with clippy::format_args

[dtolnay]

Closes #408.

Example lint:

warning: `format!` in `bail!` args
 --> src/main.rs:5:5
  |
5 |     bail!("{}", format!("something failed at {}", Location::caller()));
  |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  |
  = help: combine the `format!(..)` arguments with the outer `bail!(..)` call
  = help: or consider changing `format!` to `format_args!`
  = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#format_in_format_args
  = note: `#[warn(clippy::format_in_format_args)]` on by default