add SlidingCookie plug

[mwri]

Adds a SlidingCookie plug which implements a sliding window/session behaviour, allowing the remember_me cookie to be replaced before it expires.

Configure (or use plug option) like so:

config :my_app, MyApp.Guardian,
  ttl: {10, :minutes},
  sliding_cookie: {5, :minutes}

Add to Guardian pipeline, for example:

  plug Guardian.Plug.VerifyCookie
  plug Guardian.Plug.SlidingCookie
  plug Guardian.Plug.LoadResource
  plug Holco.Web.Plug.PostAuth

Provide sliding_cookie callback implementation:

defmodule MyAppWeb.Guardian do
  use Guardian, otp_app: :my_app
  def subject_for_token(user_rec, claims), do: ......
  def resource_from_claims(claims), do ......

  def sliding_cookie(claims, resource, opts) do
    {:ok, %{"new_claim" => "whatevs"}}
  end
end

[codecov-io]

Codecov Report

Merging #616 into master will increase coverage by 0.2%. The diff coverage is 91.3%.

@@            Coverage Diff            @@
##           master     #616     +/-   ##
=========================================
+ Coverage   86.32%   86.52%   +0.2%     
=========================================
  Files          21       22      +1     
  Lines         424      438     +14     
=========================================
+ Hits          366      379     +13     
- Misses         58       59      +1

Impacted Files	Coverage Δ
lib/guardian/plug/verify_cookie.ex	90% <ø> (-1.31%)	:arrow_down:
lib/guardian/token/jwt.ex	85.71% <100%> (+0.34%)	:arrow_up:
lib/guardian/plug.ex	85.18% <100%> (+0.56%)	:arrow_up:
lib/guardian.ex	89.28% <83.33%> (-0.72%)	:arrow_down:
lib/guardian/plug/sliding_cookie.ex	92.3% <92.3%> (ø)

---

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[mwri]

Ah ha. Fixed formatting :)

[mwri]

Rebased on master and added :halt plug option following merge of #617. I'm not sure it really ever makes sense not to halt when the implementation is broken, but I think it's nice to be entirely consistent with the other plugs.

[mwri]

Sorry, I don't know how I accidentally closed it there, must have hit a key when the button was highlighted or something.

[mwri]

Agreed changes completed, I think.

[Hanspagh]

Would love to have a view from one other @ueberauth/core and a rebase from you @mwri

[mwri]

Rebased.

[mwri]

Cheers. Always a pleasure to contribute a well received PR.