feat: configure conn halt

[hauptbenutzer]

Hi!

I finally got around to putting this PR together. It addresses the discussion in #401.
The halt option is added to all relevant plugs and defaults to true, as to not break any current behaviour. Let me know what you think.

Cheers!

[codecov-io]

Codecov Report

Merging #617 into master will increase coverage by 0.09%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #617      +/-   ##
==========================================
+ Coverage   86.22%   86.32%   +0.09%     
==========================================
  Files          21       21              
  Lines         421      424       +3     
==========================================
+ Hits          363      366       +3     
  Misses         58       58

Impacted Files	Coverage Δ
lib/guardian/plug/load_resource.ex	86.66% <100%> (ø)	⬆️
lib/guardian/plug/ensure_authenticated.ex	88.88% <100%> (ø)	⬆️
lib/guardian/plug/verify_session.ex	94.44% <100%> (ø)	⬆️
lib/guardian/plug.ex	84.61% <100%> (+0.61%)	⬆️
lib/guardian/plug/verify_header.ex	70.83% <100%> (ø)	⬆️
lib/guardian/plug/verify_cookie.ex	91.3% <100%> (ø)	⬆️
lib/guardian/plug/ensure_not_authenticated.ex	80% <100%> (ø)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 38a5558...e1c1b36. Read the comment docs.

[yordis]

@hauptbenutzer thank you very much. Would you mind adding a new entry to the CHANGELOG file following the previous examples?

[hauptbenutzer]

@yordis Like this? Not really sure what an appropriate version bump would be.

[yordis]

@hauptbenutzer yeah that was totally fine