Guard against malformed configuration

[bkeepers]

https://sentry.io/probot/stale/issues/307099740/ is an error caused by an installation that has an array value for staleLabel, which is not expected. In this case, passing that parameter on to the GitHub API doesn't cause any harm, but there could be other instances where it does.

We should do an audit of all config values to ensure they're properly checked before being used in an API request.

Error: {"message":"Invalid request.\n\nFor 'properties/name', [\"wontfix\", \"duplicate\", \"invalid\"] is not a string.","documentation_url":"https://developer.github.com/v3/issues/labels/#create-a-label"}

[bkeepers]

There are a lot of errors caused by mis-configuration.

The app should validate the type of every field and fail on invalid configuration.

[stale]

Is this still relevant? If so, please comment with any updates or addition details.