Feature request: custom derives

[tbodt]

I want to be able to add custom traits to the #[derive( on Bindgen types. My specific use case is the FromBytes derive from https://docs.rs/zerocopy. This might look something like this:

bindgen::Builder::default()
    .header("input.h")
    .raw_line("use zerocopy::FromBytes;")
    .derive_custom("FromBytes")
    .generate()
    .unwrap()

[emilio]

Dupe of #1089, probably should be keyed per-type? If so this should be reasonably straight-forward. If not, we might need more information to know when something can be derived or not...