Allow textareaFields to have a required attribute

(Just like `htmlField` does)

Resolves #817. 
I would guess its for XHTML compliance(?). [`required`](http://www.w3.org/html/wg/drafts/html/master/forms.html#the-required-attribute) is a boolean attribute, so 
`<input required>`, `<input required="">`, and `<input required="required">` should all be equivalent http://www.w3.org/html/wg/drafts/html/master/infrastructure.html#boolean-attribute.

However, I personally would always write it like `<input required>`. I was just following the lead of the rest of the `Fields.hs` in using `required=""`:

https://github.com/yesodweb/yesod/blob/4d4f0c137cd83286c3c266c3be021a168bfed710/yesod-form/Yesod/Form/Fields.hs#L167
https://github.com/yesodweb/yesod/blob/4d4f0c137cd83286c3c266c3be021a168bfed710/yesod-form/Yesod/Form/Fields.hs#L112

though some of Fields.hs does use the no-attribute style:

https://github.com/yesodweb/yesod/blob/4d4f0c137cd83286c3c266c3be021a168bfed710/yesod-form/Yesod/Form/Fields.hs#L220

Happy to change it—I definitely prefer the look of no-attribute `required`.