Add build_date

[timvink]

Current behaviour is that if git is not available (due to new projects, or shallow clones in CI), there is an error advising to enable fallback_to_build_date.

Build date and revision date are actually two separate things. The correct behaviour for revision_date should be to throw an error, and then optionally silence it with parameters. (as discussed in #26)

Additionally a new build_date tag can be made available, where is it advised to also silence git errors.

[timvink]

Change of insight here. While not a super clean solution, fallback_to_build_date works.

The build_date should be a tag supplied by https://github.com/timvink/mkdocs-git-authors-plugin, where I plan to make available many different git-related tags.