bevy_lint should pass --cfg bevy_lint when run

[BD103]

Related to #88 and #109.

Automatically passing --cfg bevy_lint would let conditional compilation check whether the Bevy Linter is available or not, and would let code do this:

#[cfg_attr(bevy_lint, allow(bevy::my_lint))]

This would help get around #![register_tool(bevy)] being a nightly-only feature, since the cfg would tell the code that nightly is available.

[janhohenheim]

Good idea! I wonder how that will play along with IDE defaults though.