Added support for `placeholder` attribute

Mr. Camuso,

While playing with Jinjup, I wanted to use the HTML5 `placeholder` attribute. I noticed you didn't have support for it, so I added it. I added it for every input type, even though it doesn't work with all of them. I couldn't find a list of all the input types it does work with, so I thought it better to be too thorough than to be not thorough enough. If you were to add a placeholder to something that doesn't display one (for example, a radio button), nothing happens. No harm done.

I am submitting this pull request hoping you can merge that in with the main repository.

Thanks, and looking forward to seeing you Monday!

Tyler