This commit adds a `GitInfo` object to `Page` if `EnableGitInfo` is set.
It then also sets `Lastmod` for the given `Page` to the author date provided by Git.
The Git integrations should be fairly performant, but it adds "some time" to the build, somewhat depending on the Git history size.
If you want, you can run without during development and turn it on when deploying to the live server: `hugo --enableGitInfo`.
Fixes #2102
There are still work to be done in the case department, but that will have to be another day.
Fixes #2581
See https://github.com/spf13/viper/issues/261
