Profile arbitrary spans

I was wondering if there's a way to add arbitrary spans to puffin. It would be nice to just do the following:

```rust
let scope = puffin::profile_scope!("slow_code");

// ...

drop(scope);
```

While in a lot of scenarios function and scope profiling is sufficient, there are some performance intervals that aren't easily measured using that methodology. Say something starts in the middle of one function and ends in the middle of another for example.

Being able to insert events with arbitrary start/end times might work too, would just require timing things myself.