Fix usage writing when using custom version flag

[hhromic]

Hi @alexflint , this is a small follow-up to PR #223 :)

I realized that I forgot to update the write usage function, and it was still printing the output of Version() even if a custom --version flag spec is present in the arguments. This is now also fixed and I added more unit tests for help/usage writing.

I also took the opportunity to improve a bit the way version flag specs are scanned for both usage and help writing.

I belive that this PR now fully addresses issue #171 because, when using a custom --version flag, the builtin version string (if any) is not printed anymore in usage and errors.

Fixes #171