Automatically initialize the global context when adding one

[Terseus]

Right now we need two context managers to use the library:

• initialize the global context with global_context_initialized.
• add the context with add_global_context.

It would be great if add_global_context can manage the context automatically:

• detect if the context is initialized.
• if it isn't, initialize and save the state.
• when add_global_context ends, close the global context.

It's important to be reentrant to an add_global_context within another add_global_context doesn't close the context too soon.