Blog: add note for articles that might be outdated (#537)

Provides a custom `site.outdated_article_threshold` variable through
a Jekyll plugin.

_plugins/outdated_article_threshold.rb

@@ -0,0 +1,10 @@
+AVERAGE_SECONDS_PER_MONTH = 2628000
+
+# Add a custom `site.outdated_article_threshold` variable.
+# We use `site.time` that holds the build time as a base, but since you can't do math in
+# the template language we can't use it there directly.
+Jekyll::Hooks.register(:site, :after_init) do |site|
+  # Articles that have a date older than this at build time are considered outdated.
+  # see `_includes/outdated_article_notice.html`
+  site.config["outdated_article_threshold"] = site.time - AVERAGE_SECONDS_PER_MONTH * 18
+end