v0.1.6 be able to have mkt-prompt part of the page, html5 validation

Should this be renamed to mkt-form? 

**\<mkt-prompt\>**

![screen shot 2014-12-22 at 11 24 59 am](https://cloud.githubusercontent.com/assets/674727/5529794/2f4ce3a4-89cd-11e4-9a4a-318ae27d3fc4.png)

**\<mkt-prompt data-modal\>**

![screen shot 2014-12-22 at 11 25 36 am](https://cloud.githubusercontent.com/assets/674727/5529801/42477faa-89cd-11e4-8aae-8fc0388ada06.png)

I think I'll keep that behavior *if* there are two buttons that don't have types on them by default. I'll simplify the rules to target the butotn types rather than first-child/last-child. But I think I'll still create the cancel button and hide it. Since I have a [macro that handles both the form and the modal](https://github.com/mozilla/fireplace/pull/871/files#diff-db3acd597d22d0d5ba9900a8212def1dR31), I don't want to add if statements to check whether it's a modal or not to hide the cancel button.

The cancel button does emit a mkt-prompt-close after the fact. But I want the hard-coded behavior of the cancel button closing the modal. I don't want to rewrite the modal-closing behavior five times across Fireplace when there's no case for *not* having the cancel button closing the modal automatically. Is that what you're suggesting?