Trigger on VSTS pull requests

[asmundg]

Being able to trigger flows on pull request creation and status changes would be massively useful.

[soydachi]

 This trigger is a must! 

[hintbwHC]

Is this a duplicate of https://powerusers.microsoft.com/t5/Flow-Ideas/Trigger-When-a-user-group-is-added-to-a-pull-request-...

It would be nice to merge them so the votes on the overall idea go up instead of being spread between the same suggestion twice.