add concept of scope to store global task data

[snopoke]

Provide data to tasks without the need to pass it in directly to every call.

from taskbadger import current_scope, create_task

with current_scope() as scope:
    scope["foo"] = "bar"
    create_task("my task 1")
    create_task("my task 2")
    create_task("my task 3")