Better way to provide installation ID when running bot locally

[pllim]

When testing bot locally, I had to do extra steps like:

from baldrick.github.github_auth import repo_to_installation_id
ins_id = repo_to_installation_id('user/repo')
pr = PullRequestHandler('user/repo', pr_num, installation=ins_id)

But desired:

pr = PullRequestHandler('user/repo', pr_num)