Disabled, Readonly, Placeholder

Add .Disabled() to proxy .Attr("disabled", "disabled") and .Readonly() to proxy .Attr("readonly", "readonly") and .Placeholder("asdf") to proxy .Attr("placeholder", "asdf")

[On trello](https://trello.com/card/add-disabled-to-proxy-attr-disabled-disabled-and-readonly-to-proxy-attr-readonly-readonly-and-placeholder-asdf-to-proxy-attr-placeholder-asdf/504df3392ad570121c36c3f7/50)