feat: Add --charset flag

[ianlewis]

Fixes #415

[codecov]

Codecov Report

Merging #436 (feac2ba) into main (295aee9) will decrease coverage by 0.38%.
The diff coverage is 64.86%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #436      +/-   ##
==========================================
- Coverage   79.94%   79.57%   -0.38%     
==========================================
  Files          19       19              
  Lines        1047     1072      +25     
  Branches       14       14              
==========================================
+ Hits          837      853      +16     
- Misses        151      157       +6     
- Partials       59       62       +3     

Files Changed	Coverage Δ
internal/cmd/todos/app.go	82.19% <62.50%> (-2.84%)	⬇️
internal/scanner/scanner.go	76.55% <66.66%> (+0.08%)	⬆️
internal/walker/walker.go	60.36% <100.00%> (ø)