Feature request: allow assignments to multiple people

[jieyouxu]

Some PRs warrant multiple primary reviewers, possibly even from multiple teams. I would like to be able to r? multiple reviewers to a PR. Example, I would like the following

r? compiler
r? jieyouxu
r? libs

to assign someone from T-compiler, someone from T-libs and myself for a PR that warrants both another compiler reviewer and a libs reviewer to look at the PR.