[Gutter] Add decorateMarker method to Gutter model

src/gutter-container.coffee

@@ -7,8 +7,11 @@ Gutter = require './gutter'
 module.exports =
 class GutterContainer
   Subscriber.includeInto(this)
-  constructor: ->
+
+  # * `textEditor` The {TextEditor} to which this {GutterContainer} belongs.
+  constructor: (textEditor) ->
     @gutters = []
+    @textEditor = textEditor
     @emitter = new Emitter
 
   destroy: ->
@@ -76,3 +79,12 @@ class GutterContainer
     else
       throw new Error 'The given gutter cannot be removed because it is not ' +
           'within this GutterContainer.'
+
+  # The public interface is Gutter::decorateMarker or TextEditor::decorateMarker.
+  addGutterDecoration: (gutter, marker, options) ->
+    if gutter.name is 'line-number'
+      options.type = 'line-number'
+    else
+      options.type = 'gutter'
+    options.gutterName = gutter.name
+    @textEditor.decorateMarker marker, options

src/gutter.coffee

@@ -43,6 +43,14 @@ class Gutter
   isVisible: ->
     @visible
 
+  # * `marker` (required) A Marker object.
+  # * `options` (optional) An object with the following fields:
+  #   * `class` (optional)
+  #   * `item` (optional) A model {Object} with a corresponding view registered,
+  #     or an {HTMLElement}.
+  decorateMarker: (marker, options) ->
+    @gutterContainer.addGutterDecoration this, marker, options
+
   # Calls your `callback` when the {Gutter}'s' visibility changes.
   #
   # * `callback` {Function}

src/text-editor.coffee

@@ -114,7 +114,7 @@ class TextEditor extends Model
       @emit 'scroll-left-changed', scrollLeft if includeDeprecatedAPIs
       @emitter.emit 'did-change-scroll-left', scrollLeft
 
-    @gutterContainer = new GutterContainer
+    @gutterContainer = new GutterContainer this
     @lineNumberGutter = @gutterContainer.addGutter
       name: 'line-number'
       priority: 0