Adding --show option for CI/CD processes to consume

[twcurrie]

Adds --show option referenced in these issues:

• Add --show option to print only the version number #57
• Bump2version is erroring when listing the current version and there is no value to replace in the file.  #181

I also strongly desire this feature, so I aimed for simplicity and minimal change on this add-on, and tried to match the coding style as best as possible. I defer to the maintainer should they have stronger opinions on implementation 😉 , but I believe there is strong interest in this feature overall.

[florisla]

This is long overdue, thanks for contributing this.

If I find the time, I will review this and include it in the next release.

[clbarnes]

Might it be better to use print rather than logging to output the version? That would play nicer with scripting, is simpler, and more expected, as logging writes to stderr and should be used to communicate with the user, where print uses stdout and represents a program's primary output.

[twcurrie]

@clbarnes I don't have a strong opinion on that, but it's a trivial change if desired - @florisla thoughts?

[florisla]

I agree that the --show output should go to stdout.