[Feature request] Allow `Scientist::Experiment` classes to *not* be default Scientist experiment class

I'm wondering what the original inspiration for making any class that `include`s the module the default experiment, [code here](https://github.com/github/scientist/blob/303c1f4ff98786b702507ed0c4f8cfdeb5fc7138/lib/scientist/experiment.rb#L13). Is it possible to create a class that `include`s the `Scientist::Experiment` module and **not** make it the default scientist experiment?

Looking [here](https://github.com/github/scientist#making-science-useful) it doesn't seem like it is possible.

From docs as well:
> When `Scientist::Experiment` is included in a class, it automatically sets it as the default implementation via `Scientist::Experiment.set_default`. This `set_default` call is is skipped if you include `Scientist::Experiment` in a module.

The reason I ask is because recently we ran into an issue with our Rails app:
1. We had a class that was running `Scientist` via `include` `Scientist` with the default experiment, but was **not** publishing anything, *oops*.
2. We created a new class for that `include`d `Scientist::Experiment`, but since our tests don't eager load our classes, it was a flaky test and not surfaced during our build.
3. When running in production, we eager loaded and overrode the default experiment and then surfaced issues.

So a few questions I was wondering:
1. Why default experiment, this seems a little aggressive IMO?
2. Can we actually find a way to either make default the normal behavior and have a way to pass a flag to override it?
3. Or is it that the way we were using it was wrong, and that the intention is to actually just `include Scientist` and override the default methods for that class? If so, the only reason I held back from doing so is better separation of responsibilities. It was going to look a bit messy to clog up one class's specs with experiment specs and I'd rather separate the two easily 🤔
4. Relating to point 3, is it the case that by the current implementation we can't be running multiple experiments at once?

I looked into it a bit myself, but it doesn't seem like there is a reasonably clean way. Although I think we can add a class method (something like):

```
class Foo
include Scientist::Experiment

default_scientist_experiment(false) # new
end
```

What do y'all think? If so - I can try to take a stab at it.

Bắt đầu với lib/scientist/experiment.rb tại logic inclusion được liên kết và phần “Making science useful” trong README. Xem xét các câu hỏi chưa được giải quyết và hành vi lựa chọn mặc định hiện tại trước khi xác định semantics mong muốn. Công việc được xem là hoàn tất khi có một cách đã được thống nhất và kiểm thử để các lớp experiment được include không trở thành mặc định, cùng với việc cập nhật mọi tài liệu cần thiết.

Do mô hình lập chỉ mục viết ra từ nội dung của issue.