Method to wait at a point

It would be nice to have a method to wait at a specific point.

For example, right now I do:
```cpp
auto t = tweeny::from(130).to(130).during(300).to(80).during(400);
```

But it would be better if I could do:
```cpp
auto t = tweeny::from(130).wait(300).to(80).during(400);
```