Add GitHub link parser to Markdown

[Xety]

No description provided.

[codecov]

Codecov Report

Merging #78 (89ef296) into master (0060321) will decrease coverage by 0.04%.
The diff coverage is 80.48%.

@@             Coverage Diff              @@
##             master      #78      +/-   ##
============================================
- Coverage     83.40%   83.35%   -0.05%     
- Complexity      677      689      +12     
============================================
  Files           147      153       +6     
  Lines          2543     2584      +41     
============================================
+ Hits           2121     2154      +33     
- Misses          422      430       +8     

Impacted Files	Coverage Δ
app/Models/DiscussConversation.php	78.46% <ø> (ø)
...Models/Presenters/DiscussConversationPresenter.php	83.33% <ø> (ø)
app/Markdown/GithubCommit/GithubCommitRenderer.php	50.00% <50.00%> (ø)
...wn/GithubPullRequest/GithubPullRequestRenderer.php	50.00% <50.00%> (ø)
...pp/Markdown/GithubCommit/GithubCommitExtension.php	100.00% <100.00%> (ø)
app/Markdown/GithubCommit/GithubCommitParser.php	100.00% <100.00%> (ø)
...n/GithubPullRequest/GithubPullRequestExtension.php	100.00% <100.00%> (ø)
...down/GithubPullRequest/GithubPullRequestParser.php	100.00% <100.00%> (ø)

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