feat: support skip frontmatter

[yeonjuan]

fixes #291

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 98.63%. Comparing base (d367b20) to head (290698f).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #301      +/-   ##
==========================================
+ Coverage   98.61%   98.63%   +0.01%     
==========================================
  Files          69       70       +1     
  Lines        2026     2048      +22     
  Branches      558      562       +4     
==========================================
+ Hits         1998     2020      +22     
  Misses         28       28              

Flag	Coverage Δ
unittest	98.63% <100.00%> (+0.01%)	⬆️

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Files with missing lines	Coverage Δ
packages/parser/lib/frontmatter.js	100.00% <100.00%> (ø)

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