Handle priority in when inserting style elements in style manager

spec/style-manager-spec.coffee

@@ -64,3 +64,17 @@ describe "StyleManager", ->
           "a {color: green}"
           "a {color: blue}"
         ]
+
+    describe "when a priority parameter is specified", ->
+      it "inserts the style sheet based on the priority", ->
+        manager.addStyleSheet("a {color: red}", priority: 1)
+        manager.addStyleSheet("a {color: blue}", priority: 0)
+        manager.addStyleSheet("a {color: green}", priority: 2)
+        manager.addStyleSheet("a {color: yellow}", priority: 1)
+
+        expect(manager.getStyleElements().map (elt) -> elt.textContent).toEqual [
+          "a {color: blue}"
+          "a {color: red}"
+          "a {color: yellow}"
+          "a {color: green}"
+        ]

src/style-manager.coffee

@@ -125,7 +125,7 @@ class StyleManager
     new Disposable => @removeStyleElement(styleElement)
 
   addStyleElement: (styleElement) ->
-    {sourcePath, group} = styleElement
+    {sourcePath, group, priority} = styleElement
 
     if group?
       for existingElement, index in @styleElements
@@ -133,6 +133,13 @@ class StyleManager
           insertIndex = index + 1
         else
           break if insertIndex?
+
+    if priority?
+      for existingElement, index in @styleElements
+        if existingElement.priority > priority
+          insertIndex = index
+          break
+
     insertIndex ?= @styleElements.length
 
     @styleElements.splice(insertIndex, 0, styleElement)