feat: Add --exclude and --exclude-dir

[ianlewis]

Fixes #63

[codecov]

Codecov Report

Merging #372 (c2beecf) into main (97e3608) will increase coverage by 0.00%.
The diff coverage is 81.25%.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #372   +/-   ##
=======================================
  Coverage   79.90%   79.90%           
=======================================
  Files          19       19           
  Lines        1015     1045   +30     
  Branches       14       14           
=======================================
+ Hits          811      835   +24     
- Misses        147      151    +4     
- Partials       57       59    +2     

Files Changed	Coverage Δ
internal/cmd/todos/app.go	85.02% <72.72%> (-2.05%)	⬇️
internal/walker/walker.go	60.36% <100.00%> (+2.57%)	⬆️