Bumpver

- Fix: [use default date values][gh_i172].

When parsing the current version, if it doesn't specify anyt
date part, (such as is the case for e.g. SemVer), then use the
current date to populate default parts.

This enables updating YYYY patterns in copyright headers even
for projects that don't use a CalVer pattern.

Thank you [Benjamin Depardon (bdepardo)][gh_bdepardo] for
finding and reporting this issue.

[gh_i172]: https://github.com/mbarkhau/bumpver/issues/172
[gh_bdepardo]: https://github.com/bdepardo

- Add: [flags to override vcs options][gh_i168] for `bumpver update`

[gh_i168]: https://github.com/mbarkhau/bumpver/issues/168

Thank you to Timo Ludwig timoludwig for this contribution.

- Add: [`--commit-message` argument][gh_i162] for `bumpver update`

[gh_i162]: https://github.com/mbarkhau/bumpver/issues/162

- Fix: Build from source on windows.

- Fix [github157][gh_i157]: Improve error messages.
- Fix [github158][gh_i158]: Clarify `PYTAGNUM` "part"

[gh_i157]: https://github.com/mbarkhau/bumpver/issues/157
[gh_i158]: https://github.com/mbarkhau/bumpver/issues/158

Thank you to Julien Palard JulienPalard for testing and feedback.

- Add `-e/--env` option to support shell script automation.
- Fix [github151][gh_i151]: invalid increment of `TAGNUM` when `TAG=final` is set.

[gh_i151]: https://github.com/mbarkhau/bumpver/issues/151

Thank you to Dave Wapstra dwapstra for your contributions.