Auto merge of #14383 - Razican:column-rule-style, r=Wafflespeanut

Stylo: implement column-rule-style

Implemented `column-rule-style` in stylo.

- [x] `./mach build -d` does not report any errors
- [x] `./mach test-tidy` does not report any errors
- [x] These changes fix #13868

- [ ] There are tests for these changes OR
- [x] These changes do not require tests because it's a style addition and I added the screenshot.

Screenshot:
![screenshot from 2016-11-27 21-59-38](https://cloud.githubusercontent.com/assets/597469/20652731/0a7ccbfc-b4ff-11e6-8e4c-1547897bcf25.png)

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components/style/properties/longhand/column.mako.rs

@@ -288,3 +288,9 @@ ${helpers.predefined_type("-moz-column-rule-color", "CSSColor",
 // https://drafts.csswg.org/css-multicol-1/#column-span
 ${helpers.single_keyword("column-span", "none all",
                          products="none", animatable=False)}
+
+${helpers.single_keyword("column-rule-style",
+                         "none hidden dotted dashed solid double groove ridge inset outset",
+                         products="gecko",
+                         gecko_constant_prefix="NS_STYLE_BORDER_STYLE",
+                         animatable=False)}