Added an explicit option to avoid scanning other files

[tkav]

As per issue #9, the action was taking a long time to complete as it scanned through other files. I have added an explicit option to only change the version_file without scanning other files. This reduced the action time for my repository from 8 minutes to 15 seconds.

- name: Bump version
        uses: tkav/bump-version@explicit-option
        with:
            version_file: ./VERSION
            github_token: ${{ secrets.GITHUB_TOKEN }}
            explicit: 1
        env:
          GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}