Add back task run key

[ahuang11]

Summary

import time
import distributed
from prefect_dask import DaskTaskRunner
from prefect import flow, task

N = 100

@task()
def f(x):
    time.sleep(1)
    return 2 * x

def main():
    start = time.time()
    tasks = f.map(range(N))
    submitted = time.time()
    return sum([t.result() for t in tasks])

flow(task_runner=DaskTaskRunner())(main)()

Relevant Issue(s)

Closes #27

Checklist

• Summarized PR's changes in CHANGELOG.md