Add support for deriving Default

[GnomedDev]

trafficstars

I would love to migrate some libraries from the unmaintained derivative to this crate, but it is missing deriving Default with custom values. If the "custom bound" functionality from https://mcarton.github.io/rust-derivative/latest/Default.html could be implemented that would be fabulous.

[JelteF]

Sounds like a good derive to also support in derive_more. Feel free to contribute a PR for it.

[JelteF]

Regarding custom bounds: so far we've been able to avoid supporting that for other derives by actually generating the correct bounds. If that's possible here too that would be great.

[JelteF]

This is very similar to #311 btw, only for different traits.

[ModProg]

https://lib.rs/crates/smart-default could be inspiration for the api