Option for toggling html output

Here's a simple option called `writehtml` (defaults to true) for controlling whether or not trumpet dumps out html. There are some use cases for trumpet where outputting html isn't useful.

Supposedly it might be possible to replace this option with a stream that can filter out all html elements, but the problem is that the wildcard css selectors don't catch `<!-- comments -->` and `<!doctypes>` I think.

Anyway, there's a new test and it passes and the other tests aren't broken.