[Developer] Improve merge PR script to acknowledge co-authors

[asfimport]

The Apache Spark PR merge tool supports lead/co-author acknowledgement that shows up in the GitHub UI, we should try to follow this

https://github.com/apache/spark/blob/master/dev/merge_spark_pr.py

example commit apache/spark@5a7aa6f

Reporter: Wes McKinney / @wesm
Assignee: Wes McKinney / @wesm

PRs and other links:

• GitHub Pull Request #4882

_{Note: This issue was originally created as ARROW-5716. Please see the migration documentation for further details.}

[asfimport]

Wes McKinney / @wesm:
Issue resolved by pull request 4882
#4882