hn: add page

pages/common/hn.md

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+# hn
+
+> Command-line interface for Hacker News.
+
+- View stories on Hacker News:
+
+`hn`
+
+- View _number_ of stories on Hacker News:
+
+`hn --limit {{number}}`
+
+- View stories on Hacker News, and keep the list open after selecting a link:
+
+`hn --keep-open`
+
+- View stories on Hacker News sorted by submission date:
+
+`hn --latest`