ci: Update GH pages manually

I can't figure out why Travis stopped working, and I feel bad
freeloading off their compile time anyway, so here's a script to update
the GitHub pages by hand

update-gh-pages.sh

@@ -0,0 +1,31 @@
+#!/bin/bash
+
+set -euxo pipefail
+
+SRCDIR="$(cd "$(dirname "${BASH_SOURCE[0]}")" >/dev/null 2>&1 && pwd)"
+TEMPDIR=/tmp/napkin-dist
+
+cd "$SRCDIR"
+latexmk
+
+if [ ! -d "$TEMPDIR" ]; then
+  mkdir -p "$TEMPDIR"
+fi
+if [ ! -d "$TEMPDIR"/.git ]; then
+  cp -r .git/ "$TEMPDIR"
+fi
+
+cd $TEMPDIR
+git checkout gh-pages
+
+mkdir -p "$TEMPDIR"/asy
+
+cd "$SRCDIR"
+cp Napkin.log "$TEMPDIR"
+cp Napkin.pdf "$TEMPDIR"
+cp asy/*.asy "$TEMPDIR"/asy
+
+cd "$TEMPDIR"
+tree -H '.' -I "index.html" -D --charset utf-8 -T "An Infinitely Large Napkin" >index.html
+git commit -a --amend -m "GitHub pages deploy from $(hostname) at $(date --utc)"
+git diff --stat HEAD origin/gh-pages