layout: Implement text-align: justify and text-justify per

CSS-TEXT-3 § 7.3.

`text-justify: distribute` is not supported.

The behavior of `text-justify: none` does not seem to match what Firefox
and Chrome do, but it seems to match the spec.

Closes #213.

components/style/properties/mod.rs.mako

@@ -1218,6 +1218,9 @@ pub mod longhands {
 
     ${single_keyword("text-overflow", "clip ellipsis")}
 
+    // TODO(pcwalton): Support `text-justify: distribute`.
+    ${single_keyword("text-justify", "auto none inter-word")}
+
     ${new_style_struct("Text", is_inherited=False)}
 
     <%self:longhand name="text-decoration">