Call setCursorBufferPosition only if this method exists

@kevinsawicki:
That way in the case where a custom editor is opened that isn't a text editor,
it won't blow up if the custom editor doesn't implement setCursorBufferPosition.

src/workspace.coffee

@@ -456,7 +456,7 @@ class Workspace extends Model
         pane.activateItem(item)
         pane.activate() if activatePane
         if options.initialLine? or options.initialColumn?
-          item.setCursorBufferPosition([options.initialLine, options.initialColumn])
+          item.setCursorBufferPosition?([options.initialLine, options.initialColumn])
         index = pane.getActiveItemIndex()
         @emit "uri-opened"
         @emitter.emit 'did-open', {uri, pane, item, index}