Support transforming keys containing invalid characters

[cookbr]

Related: #9

A webhook setup in Azure DevOps that's triggered by a work item being updated produces a response where the keys can contain characters which prevent usage within data variables in a Slack workflow; refer to the screenshot below, (left is from Slack, right is a sample event from the webhook).

flat provides a transformKey option which can provide a means to removing or replacing invalid characters to permit usage as data variables.