Derive Display

A derive for the Display trait.

```rust

[macrouse] extern crate displayderive;

[derive(Display)]

[display(fmt = "Error code: {}", code)]

struct RecordError { code: u32, }

[derive(Display)]

enum EnumError { #[display(fmt = "Error code: {}", code)] StructVariant { code: i32, }, #[display(fmt = "Error: {}", _0)] TupleVariant(&'static str), #[display(fmt = "An error has occurred.")] UnitVariant, } ```