Formatting of Julia code in Markdown files

[fredrikekre]

Extend Runic to format Julia code blocks inside Markdown (.md) files. The file dispatch is by extension: runic foo.md (or Runic.format_file("foo.md")) formats only the embedded Julia code blocks (same rules as --docstrings), leaving the surrounding prose untouched. Stdin can be routed through the Markdown formatter by passing --stdin-filename=*.md.

Directory walks are controlled by a new --extensions=<list> flag (default jl, e.g. --extensions=jl,md to walk both). Explicit file paths always bypass this filter. --lines is supported with block- granular semantics: any code block whose line range overlaps a --lines range is formatted in full; blocks outside are left untouched.

Closes #197.

[codecov]

Codecov Report

❌ Patch coverage is 91.93548% with 5 lines in your changes missing coverage. Please review.
✅ Project coverage is 92.85%. Comparing base (bae57a5) to head (069fc0f).
⚠️ Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
src/runestone.jl	69.23%	4 Missing ⚠️
src/main.jl	97.56%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #197      +/-   ##
==========================================
- Coverage   92.88%   92.85%   -0.03%     
==========================================
  Files           8        8              
  Lines        4032     4073      +41     
==========================================
+ Hits         3745     3782      +37     
- Misses        287      291       +4     

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