Feature to add dynamically new data into the global context

Hello,

I needed for my use case to add dynamically data to the global context so I did something like this:
```ruby
Scrolls.global_context(Scrolls.global_context.merge(process_name: @process_name))
```

But it wasn't thread safe and I felt like it could be a pretty common use case so I added the functionality directly into Scrolls with the methods add_global_context, let me know if you think it could be useful and if I should change it.

Thanks !