Add option to count errors if files were bumped to true

[vinistock]

Add --count-errors option to bump, which is similar to a dry run, but prints the number of total errors if all files were bumped to true. No changes are made to the files.

This is useful for measuring typing progress in big code bases where addressing a specific signature or error doesn't unlock the file being typed: true, but already reduces the remaining errors to fix.

This is how the output looks. (bspoom is just an alias for bin/spoom).

[Morriar]

Thanks for indulging me! 🙏

[Morriar]

@vinistock I just realized we should also add an entry in the README about this option: https://github.com/Shopify/spoom/blob/master/README.md#change-the-sigil-used-in-files