Update markdown preview when markdown buffer is saved

src/packages/markdown-preview/lib/markdown-preview.coffee

@@ -13,6 +13,8 @@ module.exports =
       console.warn("Can not render markdown for #{item.getUri()}")
       return
 
+    activePane.on 'core:save', => @show()
+
     editSession = item
     if nextPane = activePane.getNextPane()
       if preview = nextPane.itemForUri("markdown-preview:#{editSession.getPath()}")

src/packages/markdown-preview/spec/markdown-preview-spec.coffee

@@ -65,3 +65,15 @@ describe "MarkdownPreview package", ->
           expect(pane2.getItems()).toHaveLength 2
           expect(pane2.activeItem).toBe preview
           expect(pane1).toMatchSelector(':has(:focus)')
+
+      describe "when a buffer is modified and saved after a preview item has already been created", ->
+        it "updates the existing preview item", ->
+          rootView.getActiveView().trigger 'markdown-preview:show'
+          [pane1, pane2] = rootView.getPanes()
+          preview = pane2.activeItem
+          pane1.focus()
+
+          preview.fetchRenderedMarkdown.reset()
+          pane1.saveActiveItem = () ->
+          pane1.trigger("core:save")
+          expect(preview.fetchRenderedMarkdown).toHaveBeenCalled()