Everything's filename already matched the slug I wanted, so there was no
need to be manually-overriding some of them like this when the title
didn't match.

This creates a new Pelican plugin that will set the modified date on
each article (blog post) and page (docs page) based on the last time
that the source file was modified in git. This makes it so that it's no
longer necessary to remember to update the Modified: line in each file
whenever it's edited.