Consider changing the non_snake_case lint to skip names of #[no_mangle] extern functions

Since the function is tagged `#[no_mangle]` the exact name presumably has to exactly match something that another system is expecting, which may not be snake case. For example, JNI bindings will look something like

``` rust
#[no_mangle]
unsafe extern "C" fn Java_package_name_ClassName_methodName(...) { ... }
```
