Update format script for new gofmt

gofmt no longer supports `tabs` and `tabwidth`, but we still require
exactly-4-space tabs to preserve the narrow layout on gobyexample.com,
so re-implement this functionality with sed.

tools/format

@@ -0,0 +1,10 @@
+#!/bin/bash
+
+set -eo pipefail
+
+paths=$(ls tools/*.go examples/*/*.go)
+
+for path in $paths; do
+  gofmt -w=true $path
+  sed -i '' -e 's/	/    /g' $path
+done