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/server.lua

@@ -43,11 +43,6 @@ end
 
 -- Builds the Go binary
 M.build = function(override)
-  if not u.has_delta() then
-    vim.notify("Please install delta to use gitlab.nvim!", vim.log.levels.ERROR)
-    return
-  end
-
   local file_path = u.current_file_path()
   local parent_dir = vim.fn.fnamemodify(file_path, ":h:h:h:h")
   state.settings.bin_path = parent_dir