Allow customizing HTML attributes for alert title

[robinst]

Fixes #426.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 94.57%. Comparing base (41586dd) to head (8df5061).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff            @@
##               main     #427   +/-   ##
=========================================
  Coverage     94.57%   94.57%           
  Complexity      374      374           
=========================================
  Files           150      150           
  Lines          5018     5018           
  Branches        720      720           
=========================================
  Hits           4746     4746           
  Misses          145      145           
  Partials        127      127           

Files with missing lines	Coverage Δ
...ext/gfm/alerts/internal/AlertHtmlNodeRenderer.java	94.73% <100.00%> (ø)

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