feat: Add option for setting date pattern

[backdround]

It's an ability to set custom title's date pattern.

with github-action:

      - name: panvimdoc
        uses: kdheepak/panvimdoc@main
        with:
          vimdoc: improved-search.nvim
          titledatepattern: "%Y"

with command line:

../panvimdoc/panvimdoc.sh --project-name "improved-search.nvim" --input-file ./README.md --title-date-pattern "%Y"

I tested it manually with panvimdoc.sh and with action. It works with and without the flag as expected.

Sorry for the late request

[kdheepak]

This looks great! Do you think you can add at least one test for this too? If not no worries, I'll add one later and merge.