added fixture: placeholder

[ruscoder]

Use:

[akatov]

hmm, not sure it makes sense to have a simple placeholder like this.
Given this is a full contenteditable div with all sorts of css stylings possible I might want something similar for a placeholder. Not sure what the best way to do it would be though...

[b1rdex]

@akatov here is the best way: http://stackoverflow.com/a/18368720

[akatov]

I meant not just CSS stylings, I also meant the inclusion of arbitrary html inside the placeholder which doesn't seem to be possible in the solution from stackoverflow, as < and > get escaped. It may work in @ruscoder's version, however, I won't be merging this without any test fixtures!
In any case, I wonder whether it is better to bind the placeholder value to another model instead of the contents of an attribute value. Thoughts?