decorators 0.1.4

A macro for generating decorator methods
Documentation 
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# A Procedural Macro for Decorators
The Decorator macro generates a decorator method for each field, which is marked with `#[dec]` in front.
Additionaly a field of type `Option<T>` can be marked with `#[opt_dec]`. This generates a decorator method that will set the value to `Some(t)`.
## Example
```rust
#[derive(Decorator)]
struct Widget {
    #[dec]
    width: u32,
    #[dec]
    height: u32,

    #[opt_dec]
    background_color: Option<RGBA>,
}
```
Generates into:
```rust
struct Widget {
    width: u32,
    height: u32,
    background_color: Option<RGBA>,
}
impl Widget {
    pub fn width(self, width: u32) -> Self {
        Self {
            width,
            ..self
        }
    }
    pub fn height(self, height: u32) -> Self {
        Self {
            height,
            ..self
        }
    }
    pub fn background_color(self, background_color: RGBA) -> Self {
        Self {
            background_color: Some(background_color),
            ..self
        }
    }
}
```
Which can be used like:
```rust
let w = some_widget.width(10).height(20);
assert_eq!(w, Widget {width: 10, height: 20, background_color: None});
```