Feature idea - enumeration variant matching

Hey 👋🏾

I really like the fluid syntax of this library. While working with it, a few assertions would be great to have. Some of them are for ergonomics when working with enumeration variants. Like simple matching. So I could imagine for example something like:

```rust
enum Color {
RED,
GREEN,
BLUE,
RGB(u8, u8, u8),
}

assert_that!(Color::RED).matches(Color::RED);
assert_that!(Color::RGB(255, 0, 0)).matches(Color::RGB(_, _, _));
assert_that!(Color::RGB(255, 0, 0)).matches(Color::RGB(_, _, 0));
```

With the "default" API you can write these simply with `assert!(matches!(...))`. But then we have the syntax of the `assertor` it's `assert_that!` mixed with the other. Which does not read and feel great. So I though it might be easy enough to add this? 🙈

In any case: thanks for this library. Like it so far!