Post-merge reminder
I noticed that I often need to do some action after some MR is merged. It would be cool to be able to add special markdown block to a description that will be sent to my email after this particular MR is merged. Something like
```
[post-merge]: [@user1 @user2] Notify a customer that this is already merged
```
Possible use cases:
* Update some customers that are requested this feature or bugfix but they are not mentioned in this MR.
* Remind to create some issue after merging (for me it was the case several times)
* Prepare production servers for future deploy
* Something else...
/cc @JobV @victorwu @smcgivern
issue