Follow-up: Require Different Reviewers

This plugin previously only supported the Delta reviewer. Thanks to work
from @mrparalon it now supports Diffview also. This MR adjusts the
requirements to account for this, and the README.

It also addresses a small bug in the original implementation regarding
an async file opening action, and applies formatting to the diffview
file consistent with the rest of the project

lua/gitlab/async.lua

@@ -3,6 +3,7 @@
 local server = require("gitlab.server")
 local job    = require("gitlab.job")
 local state  = require("gitlab.state")
+local u      = require("gitlab.utils")
 
 local M      = {}