Placeholder text

To improve usability I added the ability to add placeholder text to the editable area.
`#7a8288` looks a little bit darker than other placeholder text. (e.g. browser default placeholder text for `input` element)
How about to change this `#a9a9a9` as others.
Ref: https://github.com/adobe/webkit/blob/master/LayoutTests/fast/forms/placeholder-position-expected.txt