line-clamp to take message also

[amitu]

Currently we have implemented line-clamp as optional integer. As per the spec line-clamp can take both integer and an optional message to show when clamping lines.

li {
  line-clamp: 5 "… (continued on next page)";
}

We currently accept this:

-- ftd.text: some text
line-clamp: 5

Where line-clamp is optional integer.

line-clamp is actually a shorthand for CSS properties, max-lines, block-elipses and discard. Maybe we should drop line-clamp in favour of these three?