Rework syntax of macro

Mentioned in https://github.com/binary-banter/test-each-file/issues/11

Following candidates for syntax:
```
test_each_file! {
    path: "./tests",
    attributes: "#[ignore]",
    async: true
}
```

```
#[test_each_file {
    path: "./tests",
    attributes: "#[ignore]",
}]
fn test_function(input: &str) {
    
}
```

This syntax is a lot easier to parse & a lot easier to read