v3.1.0

• Added pullapprove.group.requested_reviewers and pullapprove.group.unrequested_reviewers events
• Added text_list function for more friendly printing of lists in templates
• Added user.mention and users.mentions for prefixing with @ in templates
• Template usage errors appear in status rather than "Internal server error"
• Added users_* fields to group output for more clarity
• Fixed review requests not factoring in existing pending requests
• Better automatic string representations of objects in template output