BREAKING CHANGE: Delta Pager + Large Refactor (#43)

BREAKING CHANGE: This MR addresses an underlying issue with the original implementation in regards to detecting line numbers for comments. 

As such, this is a major breaking change. The setup function signature has changed, please review the `README.md` for the new arguments. The delta pager has also been added as a dependency: https://github.com/dandavison/delta

There will be future work to implement a native solution for parsing changes and line numbers.

lua/gitlab/async.lua

@@ -0,0 +1,67 @@
+-- This module is responsible for calling APIs in sequence. It provides
+-- an abstraction around the APIs that lets us ensure state.
+local server = require("gitlab.server")
+local job    = require("gitlab.job")
+local state  = require("gitlab.state")
+
+local M      = {}
+
+Async        = {
+  cb = nil
+}
+
+function Async:new(o)
+  o = o or {}
+  setmetatable(o, self)
+  self.__index = self
+  return o
+end
+
+function Async:init(cb)
+  self.cb = cb
+end
+
+function Async:fetch(dependencies, i)
+  if i > #dependencies then
+    self:cb()
+    return
+  end
+
+  local dependency = dependencies[i]
+
+  -- Do not call endpoint unless refresh is required
+  if state[dependency.state] ~= nil and not dependency.refresh then
+    self:fetch(dependencies, i + 1)
+    return
+  end
+
+  job.run_job(dependency.endpoint, "GET", dependency.body, function(data)
+    state[dependency.state] = data[dependency.key]
+    self:fetch(dependencies, i + 1)
+  end)
+end
+
+-- Will call APIs in sequence and set global state
+M.sequence = function(dependencies, cb)
+  return function()
+    local handler = Async:new()
+    handler:init(cb)
+
+    if not state.is_gitlab_project then
+      vim.notify("The gitlab.nvim state was not set. Do you have a .gitlab.nvim file configured?", vim.log.levels.ERROR)
+      return
+    end
+
+    if state.go_server_running then
+      handler:fetch(dependencies, 1)
+      return
+    end
+
+    server.start_server(function()
+      state.go_server_running = true
+      handler:fetch(dependencies, 1)
+    end)
+  end
+end
+
+return M