Provide a way to define given and when phases without the need of overriding a method.

It is kind of anoying to allways write:
      public override void Given()
      {
              ...
      }

It would be easier to just write something like:

```
GivenIDoSomething = () =>  ... ;
```

or

```
[Given]
void IDoSomething() => ... ;
```

or

```
Given IDoSomething = () => ... ;
```

Second way is used by MSpec.
