Booxdown is now a bash script and does some error checking

booxdown

@@ -1,4 +1,11 @@
-#!/bin/sh
+#!/usr/bin/env bash
+set -euo pipefail
+IFS=$'\n\t'
+
+if [ $# -lt 1 ] ; then
+    echo "$(basename $0) MARKDOWN-FILE" >&2
+    exit 1
+fi
 
 MDFILE=$1
 PDFFILE=$(basename $MDFILE .md).pdf