Add time between first review and last approving review

[jack-michaud]

Great plugin @hectcastro! I'm starting to use this at work to gather some metrics.

An additional metric that would be useful for me and my team is the time between the first non-author review and the last approving review.

- A) PR created
- B) (Author review, with annotations)
* C) Reviewer 1 requests changes
| D) Author writes commits and re-requests
* E) Reviewer 1 approves changes
- F) Merge

The time between C and E is what I'm most concerned with: when this duration is large, this is indicative of an area for improvement (especially when C + D repeat themselves!).

It feels to me like this is one the missing metrics between Lead Time and Time to Merge.