Add kill_switch parameter to BatchTask

[dominictarro]

Closes #26

Created a kill_switch mechanism to let BatchTask quit before running all tasks. it is useful for cases where the number of tasks to map is very high and failures may suggest the entire flow ought to cancel.

from prefect import flow, task
from prefecto.concurrency import BatchTask, CountSwitch

@task
def my_task(a, b):
    return a + b

@flow
def my_flow():
    bt = BatchTask(my_task, size=2, kill_switch=CountSwitch(3))
    # Should stop on the third batch, when 5 + "c" raises a ValueError
    bt.map(a=[1, 2, 3, 4, 5, 6, 7, 8], b=["a", 1, 2, "b", "c", 3, 4, 5])

[dominictarro]

Debating adding a traceback that relates the failed task's error/reason to the KillSwitchError.