This starts to address 1533. I still think the </p> tags
should be on their own line lined up with the start tag,
so the linter won't let through the specific example
shown in the ticket.
We were ignoring singleton tags like "input" tags in
html-grep. This was an artifact of our tokenizer originally
being built to check indentation of templates, for which
singleton tags had been a distraction. This fix actually cleans up
the template checking logic as well, since it can now rely
on the tokenizer to classify special tags and singleton tags.
The tokenizer is more complete and more specific.
