Beautifies the content of an HTML file.
$ cat dirty.html
<html><body><p>This is a test with a very long line that spans more than seventy characters and is going to be split into multiple lines.</p></body></html>
$ python3 prettyhtml.py dirty.html beautified.html
$ cat beautified.html
<html>
<body>
<p>
This is a test with a very long line that spans more than seventy characters and is going
to be split into multiple lines.
</p>
</body>
</html>