Infer dates from tags

[jaraco]

I really like what releases has done, and I'm considering adopting it in place of the technique I've ~~developed~~cobbled together, which performs textual substitution on the .rst to inject links.

One feature I'd miss dearly in a transition to releases, however, is the date inference. Rst.linker will inject a timestamp based on SCM metadata (the date the tag was created). This approach allows a maintainer to simply publish the release version, but get release dates reflected in the published docs.

Perhaps releases could offer this behavior as an opt-in feature, maybe by default if no <date> is supplied in the release directive.